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lguest: documentation V: Host

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Documentation: The Host

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Rusty Russell 2007-07-26 10:41:04 -07:00 committed by Linus Torvalds
parent dde797899a
commit bff672e630
6 changed files with 924 additions and 85 deletions
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273
drivers/lguest/core.c
View File

@ -64,11 +64,33 @@ static struct lguest_pages *lguest_pages(unsigned int cpu)
(SWITCHER_ADDR + SHARED_SWITCHER_PAGES*PAGE_SIZE))[cpu]);
}
/*H:010 We need to set up the Switcher at a high virtual address. Remember the
* Switcher is a few hundred bytes of assembler code which actually changes the
* CPU to run the Guest, and then changes back to the Host when a trap or
* interrupt happens.
*
* The Switcher code must be at the same virtual address in the Guest as the
* Host since it will be running as the switchover occurs.
*
* Trying to map memory at a particular address is an unusual thing to do, so
* it's not a simple one-liner. We also set up the per-cpu parts of the
* Switcher here.
*/
static __init int map_switcher(void)
{
int i, err;
struct page **pagep;
/*
* Map the Switcher in to high memory.
*
* It turns out that if we choose the address 0xFFC00000 (4MB under the
* top virtual address), it makes setting up the page tables really
* easy.
*/
/* We allocate an array of "struct page"s. map_vm_area() wants the
* pages in this form, rather than just an array of pointers. */
switcher_page = kmalloc(sizeof(switcher_page[0])*TOTAL_SWITCHER_PAGES,
GFP_KERNEL);
if (!switcher_page) {
@ -76,6 +98,8 @@ static __init int map_switcher(void)
goto out;
}
/* Now we actually allocate the pages. The Guest will see these pages,
* so we make sure they're zeroed. */
for (i = 0; i < TOTAL_SWITCHER_PAGES; i++) {
unsigned long addr = get_zeroed_page(GFP_KERNEL);
if (!addr) {
@ -85,6 +109,9 @@ static __init int map_switcher(void)
switcher_page[i] = virt_to_page(addr);
}
/* Now we reserve the "virtual memory area" we want: 0xFFC00000
* (SWITCHER_ADDR). We might not get it in theory, but in practice
* it's worked so far. */
switcher_vma = __get_vm_area(TOTAL_SWITCHER_PAGES * PAGE_SIZE,
VM_ALLOC, SWITCHER_ADDR, VMALLOC_END);
if (!switcher_vma) {
@ -93,49 +120,105 @@ static __init int map_switcher(void)
goto free_pages;
}
/* This code actually sets up the pages we've allocated to appear at
* SWITCHER_ADDR. map_vm_area() takes the vma we allocated above, the
* kind of pages we're mapping (kernel pages), and a pointer to our
* array of struct pages. It increments that pointer, but we don't
* care. */
pagep = switcher_page;
err = map_vm_area(switcher_vma, PAGE_KERNEL, &pagep);
if (err) {
printk("lguest: map_vm_area failed: %i\n", err);
goto free_vma;
}
/* Now the switcher is mapped at the right address, we can't fail!
* Copy in the compiled-in Switcher code (from switcher.S). */
memcpy(switcher_vma->addr, start_switcher_text,
end_switcher_text - start_switcher_text);
/* Fix up IDT entries to point into copied text. */
/* Most of the switcher.S doesn't care that it's been moved; on Intel,
* jumps are relative, and it doesn't access any references to external
* code or data.
*
* The only exception is the interrupt handlers in switcher.S: their
* addresses are placed in a table (default_idt_entries), so we need to
* update the table with the new addresses. switcher_offset() is a
* convenience function which returns the distance between the builtin
* switcher code and the high-mapped copy we just made. */
for (i = 0; i < IDT_ENTRIES; i++)
default_idt_entries[i] += switcher_offset();
/*
* Set up the Switcher's per-cpu areas.
*
* Each CPU gets two pages of its own within the high-mapped region
* (aka. "struct lguest_pages"). Much of this can be initialized now,
* but some depends on what Guest we are running (which is set up in
* copy_in_guest_info()).
*/
for_each_possible_cpu(i) {
/* lguest_pages() returns this CPU's two pages. */
struct lguest_pages *pages = lguest_pages(i);
/* This is a convenience pointer to make the code fit one
* statement to a line. */
struct lguest_ro_state *state = &pages->state;
/* These fields are static: rest done in copy_in_guest_info */
/* The Global Descriptor Table: the Host has a different one
* for each CPU. We keep a descriptor for the GDT which says
* where it is and how big it is (the size is actually the last
* byte, not the size, hence the "-1"). */
state->host_gdt_desc.size = GDT_SIZE-1;
state->host_gdt_desc.address = (long)get_cpu_gdt_table(i);
/* All CPUs on the Host use the same Interrupt Descriptor
* Table, so we just use store_idt(), which gets this CPU's IDT
* descriptor. */
store_idt(&state->host_idt_desc);
/* The descriptors for the Guest's GDT and IDT can be filled
* out now, too. We copy the GDT & IDT into ->guest_gdt and
* ->guest_idt before actually running the Guest. */
state->guest_idt_desc.size = sizeof(state->guest_idt)-1;
state->guest_idt_desc.address = (long)&state->guest_idt;
state->guest_gdt_desc.size = sizeof(state->guest_gdt)-1;
state->guest_gdt_desc.address = (long)&state->guest_gdt;
/* We know where we want the stack to be when the Guest enters
* the switcher: in pages->regs. The stack grows upwards, so
* we start it at the end of that structure. */
state->guest_tss.esp0 = (long)(&pages->regs + 1);
/* And this is the GDT entry to use for the stack: we keep a
* couple of special LGUEST entries. */
state->guest_tss.ss0 = LGUEST_DS;
/* No I/O for you! */
/* x86 can have a finegrained bitmap which indicates what I/O
* ports the process can use. We set it to the end of our
* structure, meaning "none". */
state->guest_tss.io_bitmap_base = sizeof(state->guest_tss);
/* Some GDT entries are the same across all Guests, so we can
* set them up now. */
setup_default_gdt_entries(state);
/* Most IDT entries are the same for all Guests, too.*/
setup_default_idt_entries(state, default_idt_entries);
/* Setup LGUEST segments on all cpus */
/* The Host needs to be able to use the LGUEST segments on this
* CPU, too, so put them in the Host GDT. */
get_cpu_gdt_table(i)[GDT_ENTRY_LGUEST_CS] = FULL_EXEC_SEGMENT;
get_cpu_gdt_table(i)[GDT_ENTRY_LGUEST_DS] = FULL_SEGMENT;
}
/* Initialize entry point into switcher. */
/* In the Switcher, we want the %cs segment register to use the
* LGUEST_CS GDT entry: we've put that in the Host and Guest GDTs, so
* it will be undisturbed when we switch. To change %cs and jump we
* need this structure to feed to Intel's "lcall" instruction. */
lguest_entry.offset = (long)switch_to_guest + switcher_offset();
lguest_entry.segment = LGUEST_CS;
printk(KERN_INFO "lguest: mapped switcher at %p\n",
switcher_vma->addr);
/* And we succeeded... */
return 0;
free_vma:
@ -149,35 +232,58 @@ static __init int map_switcher(void)
out:
return err;
}
/*:*/
/* Cleaning up the mapping when the module is unloaded is almost...
* too easy. */
static void unmap_switcher(void)
{
unsigned int i;
/* vunmap() undoes *both* map_vm_area() and __get_vm_area(). */
vunmap(switcher_vma->addr);
/* Now we just need to free the pages we copied the switcher into */
for (i = 0; i < TOTAL_SWITCHER_PAGES; i++)
__free_pages(switcher_page[i], 0);
}
/* IN/OUT insns: enough to get us past boot-time probing. */
/*H:130 Our Guest is usually so well behaved; it never tries to do things it
* isn't allowed to. Unfortunately, "struct paravirt_ops" isn't quite
* complete, because it doesn't contain replacements for the Intel I/O
* instructions. As a result, the Guest sometimes fumbles across one during
* the boot process as it probes for various things which are usually attached
* to a PC.
*
* When the Guest uses one of these instructions, we get trap #13 (General
* Protection Fault) and come here. We see if it's one of those troublesome
* instructions and skip over it. We return true if we did. */
static int emulate_insn(struct lguest *lg)
{
u8 insn;
unsigned int insnlen = 0, in = 0, shift = 0;
/* The eip contains the *virtual* address of the Guest's instruction:
* guest_pa just subtracts the Guest's page_offset. */
unsigned long physaddr = guest_pa(lg, lg->regs->eip);
/* This only works for addresses in linear mapping... */
/* The guest_pa() function only works for Guest kernel addresses, but
* that's all we're trying to do anyway. */
if (lg->regs->eip < lg->page_offset)
return 0;
/* Decoding x86 instructions is icky. */
lgread(lg, &insn, physaddr, 1);
/* Operand size prefix means it's actually for ax. */
/* 0x66 is an "operand prefix". It means it's using the upper 16 bits
of the eax register. */
if (insn == 0x66) {
shift = 16;
/* The instruction is 1 byte so far, read the next byte. */
insnlen = 1;
lgread(lg, &insn, physaddr + insnlen, 1);
}
/* We can ignore the lower bit for the moment and decode the 4 opcodes
* we need to emulate. */
switch (insn & 0xFE) {
case 0xE4: /* in <next byte>,%al */
insnlen += 2;
@ -194,9 +300,13 @@ static int emulate_insn(struct lguest *lg)
insnlen += 1;
break;
default:
/* OK, we don't know what this is, can't emulate. */
return 0;
}
/* If it was an "IN" instruction, they expect the result to be read
* into %eax, so we change %eax. We always return all-ones, which
* traditionally means "there's nothing there". */
if (in) {
/* Lower bit tells is whether it's a 16 or 32 bit access */
if (insn & 0x1)
@ -204,9 +314,12 @@ static int emulate_insn(struct lguest *lg)
else
lg->regs->eax |= (0xFFFF << shift);
}
/* Finally, we've "done" the instruction, so move past it. */
lg->regs->eip += insnlen;
/* Success! */
return 1;
}
/*:*/
/*L:305
* Dealing With Guest Memory.
@ -321,13 +434,24 @@ static void run_guest_once(struct lguest *lg, struct lguest_pages *pages)
: "memory", "%edx", "%ecx", "%edi", "%esi");
}
/*H:030 Let's jump straight to the the main loop which runs the Guest.
* Remember, this is called by the Launcher reading /dev/lguest, and we keep
* going around and around until something interesting happens. */
int run_guest(struct lguest *lg, unsigned long __user *user)
{
/* We stop running once the Guest is dead. */
while (!lg->dead) {
/* We need to initialize this, otherwise gcc complains. It's
* not (yet) clever enough to see that it's initialized when we
* need it. */
unsigned int cr2 = 0; /* Damn gcc */
/* Hypercalls first: we might have been out to userspace */
/* First we run any hypercalls the Guest wants done: either in
* the hypercall ring in "struct lguest_data", or directly by
* using int 31 (LGUEST_TRAP_ENTRY). */
do_hypercalls(lg);
/* It's possible the Guest did a SEND_DMA hypercall to the
* Launcher, in which case we return from the read() now. */
if (lg->dma_is_pending) {
if (put_user(lg->pending_dma, user) ||
put_user(lg->pending_key, user+1))
@ -335,6 +459,7 @@ int run_guest(struct lguest *lg, unsigned long __user *user)
return sizeof(unsigned long)*2;
}
/* Check for signals */
if (signal_pending(current))
return -ERESTARTSYS;
@ -342,77 +467,154 @@ int run_guest(struct lguest *lg, unsigned long __user *user)
if (lg->break_out)
return -EAGAIN;
/* Check if there are any interrupts which can be delivered
* now: if so, this sets up the hander to be executed when we
* next run the Guest. */
maybe_do_interrupt(lg);
/* All long-lived kernel loops need to check with this horrible
* thing called the freezer. If the Host is trying to suspend,
* it stops us. */
try_to_freeze();
/* Just make absolutely sure the Guest is still alive. One of
* those hypercalls could have been fatal, for example. */
if (lg->dead)
break;
/* If the Guest asked to be stopped, we sleep. The Guest's
* clock timer or LHCALL_BREAK from the Waker will wake us. */
if (lg->halted) {
set_current_state(TASK_INTERRUPTIBLE);
schedule();
continue;
}
/* OK, now we're ready to jump into the Guest. First we put up
* the "Do Not Disturb" sign: */
local_irq_disable();
/* Even if *we* don't want FPU trap, guest might... */
/* Remember the awfully-named TS bit? If the Guest has asked
* to set it we set it now, so we can trap and pass that trap
* to the Guest if it uses the FPU. */
if (lg->ts)
set_ts();
/* Don't let Guest do SYSENTER: we can't handle it. */
/* SYSENTER is an optimized way of doing system calls. We
* can't allow it because it always jumps to privilege level 0.
* A normal Guest won't try it because we don't advertise it in
* CPUID, but a malicious Guest (or malicious Guest userspace
* program) could, so we tell the CPU to disable it before
* running the Guest. */
if (boot_cpu_has(X86_FEATURE_SEP))
wrmsr(MSR_IA32_SYSENTER_CS, 0, 0);
/* Now we actually run the Guest. It will pop back out when
* something interesting happens, and we can examine its
* registers to see what it was doing. */
run_guest_once(lg, lguest_pages(raw_smp_processor_id()));
/* Save cr2 now if we page-faulted. */
/* The "regs" pointer contains two extra entries which are not
* really registers: a trap number which says what interrupt or
* trap made the switcher code come back, and an error code
* which some traps set. */
/* If the Guest page faulted, then the cr2 register will tell
* us the bad virtual address. We have to grab this now,
* because once we re-enable interrupts an interrupt could
* fault and thus overwrite cr2, or we could even move off to a
* different CPU. */
if (lg->regs->trapnum == 14)
cr2 = read_cr2();
/* Similarly, if we took a trap because the Guest used the FPU,
* we have to restore the FPU it expects to see. */
else if (lg->regs->trapnum == 7)
math_state_restore();
/* Restore SYSENTER if it's supposed to be on. */
if (boot_cpu_has(X86_FEATURE_SEP))
wrmsr(MSR_IA32_SYSENTER_CS, __KERNEL_CS, 0);
/* Now we're ready to be interrupted or moved to other CPUs */
local_irq_enable();
/* OK, so what happened? */
switch (lg->regs->trapnum) {
case 13: /* We've intercepted a GPF. */
/* Check if this was one of those annoying IN or OUT
* instructions which we need to emulate. If so, we
* just go back into the Guest after we've done it. */
if (lg->regs->errcode == 0) {
if (emulate_insn(lg))
continue;
}
break;
case 14: /* We've intercepted a page fault. */
/* The Guest accessed a virtual address that wasn't
* mapped. This happens a lot: we don't actually set
* up most of the page tables for the Guest at all when
* we start: as it runs it asks for more and more, and
* we set them up as required. In this case, we don't
* even tell the Guest that the fault happened.
*
* The errcode tells whether this was a read or a
* write, and whether kernel or userspace code. */
if (demand_page(lg, cr2, lg->regs->errcode))
continue;
/* If lguest_data is NULL, this won't hurt. */
/* OK, it's really not there (or not OK): the Guest
* needs to know. We write out the cr2 value so it
* knows where the fault occurred.
*
* Note that if the Guest were really messed up, this
* could happen before it's done the INITIALIZE
* hypercall, so lg->lguest_data will be NULL, so
* &lg->lguest_data->cr2 will be address 8. Writing
* into that address won't hurt the Host at all,
* though. */
if (put_user(cr2, &lg->lguest_data->cr2))
kill_guest(lg, "Writing cr2");
break;
case 7: /* We've intercepted a Device Not Available fault. */
/* If they don't want to know, just absorb it. */
/* If the Guest doesn't want to know, we already
* restored the Floating Point Unit, so we just
* continue without telling it. */
if (!lg->ts)
continue;
break;
case 32 ... 255: /* Real interrupt, fall thru */
case 32 ... 255:
/* These values mean a real interrupt occurred, in
* which case the Host handler has already been run.
* We just do a friendly check if another process
* should now be run, then fall through to loop
* around: */
cond_resched();
case LGUEST_TRAP_ENTRY: /* Handled at top of loop */
continue;
}
/* If we get here, it's a trap the Guest wants to know
* about. */
if (deliver_trap(lg, lg->regs->trapnum))
continue;
/* If the Guest doesn't have a handler (either it hasn't
* registered any yet, or it's one of the faults we don't let
* it handle), it dies with a cryptic error message. */
kill_guest(lg, "unhandled trap %li at %#lx (%#lx)",
lg->regs->trapnum, lg->regs->eip,
lg->regs->trapnum == 14 ? cr2 : lg->regs->errcode);
}
/* The Guest is dead => "No such file or directory" */
return -ENOENT;
}
/* Now we can look at each of the routines this calls, in increasing order of
* complexity: do_hypercalls(), emulate_insn(), maybe_do_interrupt(),
* deliver_trap() and demand_page(). After all those, we'll be ready to
* examine the Switcher, and our philosophical understanding of the Host/Guest
* duality will be complete. :*/
int find_free_guest(void)
{
unsigned int i;
@ -430,55 +632,96 @@ static void adjust_pge(void *on)
write_cr4(read_cr4() & ~X86_CR4_PGE);
}
/*H:000
* Welcome to the Host!
*
* By this point your brain has been tickled by the Guest code and numbed by
* the Launcher code; prepare for it to be stretched by the Host code. This is
* the heart. Let's begin at the initialization routine for the Host's lg
* module.
*/
static int __init init(void)
{
int err;
/* Lguest can't run under Xen, VMI or itself. It does Tricky Stuff. */
if (paravirt_enabled()) {
printk("lguest is afraid of %s\n", paravirt_ops.name);
return -EPERM;
}
/* First we put the Switcher up in very high virtual memory. */
err = map_switcher();
if (err)
return err;
/* Now we set up the pagetable implementation for the Guests. */
err = init_pagetables(switcher_page, SHARED_SWITCHER_PAGES);
if (err) {
unmap_switcher();
return err;
}
/* The I/O subsystem needs some things initialized. */
lguest_io_init();
/* /dev/lguest needs to be registered. */
err = lguest_device_init();
if (err) {
free_pagetables();
unmap_switcher();
return err;
}
/* Finally, we need to turn off "Page Global Enable". PGE is an
* optimization where page table entries are specially marked to show
* they never change. The Host kernel marks all the kernel pages this
* way because it's always present, even when userspace is running.
*
* Lguest breaks this: unbeknownst to the rest of the Host kernel, we
* switch to the Guest kernel. If you don't disable this on all CPUs,
* you'll get really weird bugs that you'll chase for two days.
*
* I used to turn PGE off every time we switched to the Guest and back
* on when we return, but that slowed the Switcher down noticibly. */
/* We don't need the complexity of CPUs coming and going while we're
* doing this. */
lock_cpu_hotplug();
if (cpu_has_pge) { /* We have a broader idea of "global". */
/* Remember that this was originally set (for cleanup). */
cpu_had_pge = 1;
/* adjust_pge is a helper function which sets or unsets the PGE
* bit on its CPU, depending on the argument (0 == unset). */
on_each_cpu(adjust_pge, (void *)0, 0, 1);
/* Turn off the feature in the global feature set. */
clear_bit(X86_FEATURE_PGE, boot_cpu_data.x86_capability);
}
unlock_cpu_hotplug();
/* All good! */
return 0;
}
/* Cleaning up is just the same code, backwards. With a little French. */
static void __exit fini(void)
{
lguest_device_remove();
free_pagetables();
unmap_switcher();
/* If we had PGE before we started, turn it back on now. */
lock_cpu_hotplug();
if (cpu_had_pge) {
set_bit(X86_FEATURE_PGE, boot_cpu_data.x86_capability);
/* adjust_pge's argument "1" means set PGE. */
on_each_cpu(adjust_pge, (void *)1, 0, 1);
}
unlock_cpu_hotplug();
}
/* The Host side of lguest can be a module. This is a nice way for people to
* play with it. */
module_init(init);
module_exit(fini);
MODULE_LICENSE("GPL");

118
drivers/lguest/hypercalls.c
View File

@ -28,37 +28,63 @@
#include <irq_vectors.h>
#include "lg.h"
/*H:120 This is the core hypercall routine: where the Guest gets what it
* wants. Or gets killed. Or, in the case of LHCALL_CRASH, both.
*
* Remember from the Guest: %eax == which call to make, and the arguments are
* packed into %edx, %ebx and %ecx if needed. */
static void do_hcall(struct lguest *lg, struct lguest_regs *regs)
{
switch (regs->eax) {
case LHCALL_FLUSH_ASYNC:
/* This call does nothing, except by breaking out of the Guest
* it makes us process all the asynchronous hypercalls. */
break;
case LHCALL_LGUEST_INIT:
/* You can't get here unless you're already initialized. Don't
* do that. */
kill_guest(lg, "already have lguest_data");
break;
case LHCALL_CRASH: {
/* Crash is such a trivial hypercall that we do it in four
* lines right here. */
char msg[128];
/* If the lgread fails, it will call kill_guest() itself; the
* kill_guest() with the message will be ignored. */
lgread(lg, msg, regs->edx, sizeof(msg));
msg[sizeof(msg)-1] = '\0';
kill_guest(lg, "CRASH: %s", msg);
break;
}
case LHCALL_FLUSH_TLB:
/* FLUSH_TLB comes in two flavors, depending on the
* argument: */
if (regs->edx)
guest_pagetable_clear_all(lg);
else
guest_pagetable_flush_user(lg);
break;
case LHCALL_GET_WALLCLOCK: {
/* The Guest wants to know the real time in seconds since 1970,
* in good Unix tradition. */
struct timespec ts;
ktime_get_real_ts(&ts);
regs->eax = ts.tv_sec;
break;
}
case LHCALL_BIND_DMA:
/* BIND_DMA really wants four arguments, but it's the only call
* which does. So the Guest packs the number of buffers and
* the interrupt number into the final argument, and we decode
* it here. This can legitimately fail, since we currently
* place a limit on the number of DMA pools a Guest can have.
* So we return true or false from this call. */
regs->eax = bind_dma(lg, regs->edx, regs->ebx,
regs->ecx >> 8, regs->ecx & 0xFF);
break;
/* All these calls simply pass the arguments through to the right
* routines. */
case LHCALL_SEND_DMA:
send_dma(lg, regs->edx, regs->ebx);
break;
@ -86,10 +112,13 @@ static void do_hcall(struct lguest *lg, struct lguest_regs *regs)
case LHCALL_SET_CLOCKEVENT:
guest_set_clockevent(lg, regs->edx);
break;
case LHCALL_TS:
/* This sets the TS flag, as we saw used in run_guest(). */
lg->ts = regs->edx;
break;
case LHCALL_HALT:
/* Similarly, this sets the halted flag for run_guest(). */
lg->halted = 1;
break;
default:
@ -97,25 +126,42 @@ static void do_hcall(struct lguest *lg, struct lguest_regs *regs)
}
}
/* We always do queued calls before actual hypercall. */
/* Asynchronous hypercalls are easy: we just look in the array in the Guest's
* "struct lguest_data" and see if there are any new ones marked "ready".
*
* We are careful to do these in order: obviously we respect the order the
* Guest put them in the ring, but we also promise the Guest that they will
* happen before any normal hypercall (which is why we check this before
* checking for a normal hcall). */
static void do_async_hcalls(struct lguest *lg)
{
unsigned int i;
u8 st[LHCALL_RING_SIZE];
/* For simplicity, we copy the entire call status array in at once. */
if (copy_from_user(&st, &lg->lguest_data->hcall_status, sizeof(st)))
return;
/* We process "struct lguest_data"s hcalls[] ring once. */
for (i = 0; i < ARRAY_SIZE(st); i++) {
struct lguest_regs regs;
/* We remember where we were up to from last time. This makes
* sure that the hypercalls are done in the order the Guest
* places them in the ring. */
unsigned int n = lg->next_hcall;
/* 0xFF means there's no call here (yet). */
if (st[n] == 0xFF)
break;
/* OK, we have hypercall. Increment the "next_hcall" cursor,
* and wrap back to 0 if we reach the end. */
if (++lg->next_hcall == LHCALL_RING_SIZE)
lg->next_hcall = 0;
/* We copy the hypercall arguments into a fake register
* structure. This makes life simple for do_hcall(). */
if (get_user(regs.eax, &lg->lguest_data->hcalls[n].eax)
|| get_user(regs.edx, &lg->lguest_data->hcalls[n].edx)
|| get_user(regs.ecx, &lg->lguest_data->hcalls[n].ecx)
@ -124,74 +170,126 @@ static void do_async_hcalls(struct lguest *lg)
break;
}
/* Do the hypercall, same as a normal one. */
do_hcall(lg, &regs);
/* Mark the hypercall done. */
if (put_user(0xFF, &lg->lguest_data->hcall_status[n])) {
kill_guest(lg, "Writing result for async hypercall");
break;
}
/* Stop doing hypercalls if we've just done a DMA to the
* Launcher: it needs to service this first. */
if (lg->dma_is_pending)
break;
}
}
/* Last of all, we look at what happens first of all. The very first time the
* Guest makes a hypercall, we end up here to set things up: */
static void initialize(struct lguest *lg)
{
u32 tsc_speed;
/* You can't do anything until you're initialized. The Guest knows the
* rules, so we're unforgiving here. */
if (lg->regs->eax != LHCALL_LGUEST_INIT) {
kill_guest(lg, "hypercall %li before LGUEST_INIT",
lg->regs->eax);
return;
}
/* We only tell the guest to use the TSC if it's reliable. */
/* We insist that the Time Stamp Counter exist and doesn't change with
* cpu frequency. Some devious chip manufacturers decided that TSC
* changes could be handled in software. I decided that time going
* backwards might be good for benchmarks, but it's bad for users.
*
* We also insist that the TSC be stable: the kernel detects unreliable
* TSCs for its own purposes, and we use that here. */
if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) && !check_tsc_unstable())
tsc_speed = tsc_khz;
else
tsc_speed = 0;
/* The pointer to the Guest's "struct lguest_data" is the only
* argument. */
lg->lguest_data = (struct lguest_data __user *)lg->regs->edx;
/* We check here so we can simply copy_to_user/from_user */
/* If we check the address they gave is OK now, we can simply
* copy_to_user/from_user from now on rather than using lgread/lgwrite.
* I put this in to show that I'm not immune to writing stupid
* optimizations. */
if (!lguest_address_ok(lg, lg->regs->edx, sizeof(*lg->lguest_data))) {
kill_guest(lg, "bad guest page %p", lg->lguest_data);
return;
}
/* The Guest tells us where we're not to deliver interrupts by putting
* the range of addresses into "struct lguest_data". */
if (get_user(lg->noirq_start, &lg->lguest_data->noirq_start)
|| get_user(lg->noirq_end, &lg->lguest_data->noirq_end)
/* We reserve the top pgd entry. */
/* We tell the Guest that it can't use the top 4MB of virtual
* addresses used by the Switcher. */
|| put_user(4U*1024*1024, &lg->lguest_data->reserve_mem)
|| put_user(tsc_speed, &lg->lguest_data->tsc_khz)
/* We also give the Guest a unique id, as used in lguest_net.c. */
|| put_user(lg->guestid, &lg->lguest_data->guestid))
kill_guest(lg, "bad guest page %p", lg->lguest_data);
/* This is the one case where the above accesses might have
* been the first write to a Guest page. This may have caused
* a copy-on-write fault, but the Guest might be referring to
* the old (read-only) page. */
/* This is the one case where the above accesses might have been the
* first write to a Guest page. This may have caused a copy-on-write
* fault, but the Guest might be referring to the old (read-only)
* page. */
guest_pagetable_clear_all(lg);
}
/* Now we've examined the hypercall code; our Guest can make requests. There
* is one other way we can do things for the Guest, as we see in
* emulate_insn(). */
/* Even if we go out to userspace and come back, we don't want to do
* the hypercall again. */
/*H:110 Tricky point: we mark the hypercall as "done" once we've done it.
* Normally we don't need to do this: the Guest will run again and update the
* trap number before we come back around the run_guest() loop to
* do_hypercalls().
*
* However, if we are signalled or the Guest sends DMA to the Launcher, that
* loop will exit without running the Guest. When it comes back it would try
* to re-run the hypercall. */
static void clear_hcall(struct lguest *lg)
{
lg->regs->trapnum = 255;
}
/*H:100
* Hypercalls
*
* Remember from the Guest, hypercalls come in two flavors: normal and
* asynchronous. This file handles both of types.
*/
void do_hypercalls(struct lguest *lg)
{
/* Not initialized yet? */
if (unlikely(!lg->lguest_data)) {
/* Did the Guest make a hypercall? We might have come back for
* some other reason (an interrupt, a different trap). */
if (lg->regs->trapnum == LGUEST_TRAP_ENTRY) {
/* Set up the "struct lguest_data" */
initialize(lg);
/* The hypercall is done. */
clear_hcall(lg);
}
return;
}
/* The Guest has initialized.
*
* Look in the hypercall ring for the async hypercalls: */
do_async_hcalls(lg);
/* If we stopped reading the hypercall ring because the Guest did a
* SEND_DMA to the Launcher, we want to return now. Otherwise if the
* Guest asked us to do a hypercall, we do it. */
if (!lg->dma_is_pending && lg->regs->trapnum == LGUEST_TRAP_ENTRY) {
do_hcall(lg, lg->regs);
/* The hypercall is done. */
clear_hcall(lg);
}
}

176
drivers/lguest/interrupts_and_traps.c
View File

@ -14,100 +14,147 @@
#include <linux/uaccess.h>
#include "lg.h"
/* The address of the interrupt handler is split into two bits: */
static unsigned long idt_address(u32 lo, u32 hi)
{
return (lo & 0x0000FFFF) | (hi & 0xFFFF0000);
}
/* The "type" of the interrupt handler is a 4 bit field: we only support a
* couple of types. */
static int idt_type(u32 lo, u32 hi)
{
return (hi >> 8) & 0xF;
}
/* An IDT entry can't be used unless the "present" bit is set. */
static int idt_present(u32 lo, u32 hi)
{
return (hi & 0x8000);
}
/* We need a helper to "push" a value onto the Guest's stack, since that's a
* big part of what delivering an interrupt does. */
static void push_guest_stack(struct lguest *lg, unsigned long *gstack, u32 val)
{
/* Stack grows upwards: move stack then write value. */
*gstack -= 4;
lgwrite_u32(lg, *gstack, val);
}
/*H:210 The set_guest_interrupt() routine actually delivers the interrupt or
* trap. The mechanics of delivering traps and interrupts to the Guest are the
* same, except some traps have an "error code" which gets pushed onto the
* stack as well: the caller tells us if this is one.
*
* "lo" and "hi" are the two parts of the Interrupt Descriptor Table for this
* interrupt or trap. It's split into two parts for traditional reasons: gcc
* on i386 used to be frightened by 64 bit numbers.
*
* We set up the stack just like the CPU does for a real interrupt, so it's
* identical for the Guest (and the standard "iret" instruction will undo
* it). */
static void set_guest_interrupt(struct lguest *lg, u32 lo, u32 hi, int has_err)
{
unsigned long gstack;
u32 eflags, ss, irq_enable;
/* If they want a ring change, we use new stack and push old ss/esp */
/* There are two cases for interrupts: one where the Guest is already
* in the kernel, and a more complex one where the Guest is in
* userspace. We check the privilege level to find out. */
if ((lg->regs->ss&0x3) != GUEST_PL) {
/* The Guest told us their kernel stack with the SET_STACK
* hypercall: both the virtual address and the segment */
gstack = guest_pa(lg, lg->esp1);
ss = lg->ss1;
/* We push the old stack segment and pointer onto the new
* stack: when the Guest does an "iret" back from the interrupt
* handler the CPU will notice they're dropping privilege
* levels and expect these here. */
push_guest_stack(lg, &gstack, lg->regs->ss);
push_guest_stack(lg, &gstack, lg->regs->esp);
} else {
/* We're staying on the same Guest (kernel) stack. */
gstack = guest_pa(lg, lg->regs->esp);
ss = lg->regs->ss;
}
/* We use IF bit in eflags to indicate whether irqs were enabled
(it's always 1, since irqs are enabled when guest is running). */
/* Remember that we never let the Guest actually disable interrupts, so
* the "Interrupt Flag" bit is always set. We copy that bit from the
* Guest's "irq_enabled" field into the eflags word: the Guest copies
* it back in "lguest_iret". */
eflags = lg->regs->eflags;
if (get_user(irq_enable, &lg->lguest_data->irq_enabled) == 0
&& !(irq_enable & X86_EFLAGS_IF))
eflags &= ~X86_EFLAGS_IF;
/* An interrupt is expected to push three things on the stack: the old
* "eflags" word, the old code segment, and the old instruction
* pointer. */
push_guest_stack(lg, &gstack, eflags);
push_guest_stack(lg, &gstack, lg->regs->cs);
push_guest_stack(lg, &gstack, lg->regs->eip);
/* For the six traps which supply an error code, we push that, too. */
if (has_err)
push_guest_stack(lg, &gstack, lg->regs->errcode);
/* Change the real stack so switcher returns to trap handler */
/* Now we've pushed all the old state, we change the stack, the code
* segment and the address to execute. */
lg->regs->ss = ss;
lg->regs->esp = gstack + lg->page_offset;
lg->regs->cs = (__KERNEL_CS|GUEST_PL);
lg->regs->eip = idt_address(lo, hi);
/* Disable interrupts for an interrupt gate. */
/* There are two kinds of interrupt handlers: 0xE is an "interrupt
* gate" which expects interrupts to be disabled on entry. */
if (idt_type(lo, hi) == 0xE)
if (put_user(0, &lg->lguest_data->irq_enabled))
kill_guest(lg, "Disabling interrupts");
}
/*H:200
* Virtual Interrupts.
*
* maybe_do_interrupt() gets called before every entry to the Guest, to see if
* we should divert the Guest to running an interrupt handler. */
void maybe_do_interrupt(struct lguest *lg)
{
unsigned int irq;
DECLARE_BITMAP(blk, LGUEST_IRQS);
struct desc_struct *idt;
/* If the Guest hasn't even initialized yet, we can do nothing. */
if (!lg->lguest_data)
return;
/* Mask out any interrupts they have blocked. */
/* Take our "irqs_pending" array and remove any interrupts the Guest
* wants blocked: the result ends up in "blk". */
if (copy_from_user(&blk, lg->lguest_data->blocked_interrupts,
sizeof(blk)))
return;
bitmap_andnot(blk, lg->irqs_pending, blk, LGUEST_IRQS);
/* Find the first interrupt. */
irq = find_first_bit(blk, LGUEST_IRQS);
/* None? Nothing to do */
if (irq >= LGUEST_IRQS)
return;
/* They may be in the middle of an iret, where they asked us never to
* deliver interrupts. */
if (lg->regs->eip >= lg->noirq_start && lg->regs->eip < lg->noirq_end)
return;
/* If they're halted, we re-enable interrupts. */
/* If they're halted, interrupts restart them. */
if (lg->halted) {
/* Re-enable interrupts. */
if (put_user(X86_EFLAGS_IF, &lg->lguest_data->irq_enabled))
kill_guest(lg, "Re-enabling interrupts");
lg->halted = 0;
} else {
/* Maybe they have interrupts disabled? */
/* Otherwise we check if they have interrupts disabled. */
u32 irq_enabled;
if (get_user(irq_enabled, &lg->lguest_data->irq_enabled))
irq_enabled = 0;
@ -115,112 +162,197 @@ void maybe_do_interrupt(struct lguest *lg)
return;
}
/* Look at the IDT entry the Guest gave us for this interrupt. The
* first 32 (FIRST_EXTERNAL_VECTOR) entries are for traps, so we skip
* over them. */
idt = &lg->idt[FIRST_EXTERNAL_VECTOR+irq];
/* If they don't have a handler (yet?), we just ignore it */
if (idt_present(idt->a, idt->b)) {
/* OK, mark it no longer pending and deliver it. */
clear_bit(irq, lg->irqs_pending);
/* set_guest_interrupt() takes the interrupt descriptor and a
* flag to say whether this interrupt pushes an error code onto
* the stack as well: virtual interrupts never do. */
set_guest_interrupt(lg, idt->a, idt->b, 0);
}
}
/*H:220 Now we've got the routines to deliver interrupts, delivering traps
* like page fault is easy. The only trick is that Intel decided that some
* traps should have error codes: */
static int has_err(unsigned int trap)
{
return (trap == 8 || (trap >= 10 && trap <= 14) || trap == 17);
}
/* deliver_trap() returns true if it could deliver the trap. */
int deliver_trap(struct lguest *lg, unsigned int num)
{
u32 lo = lg->idt[num].a, hi = lg->idt[num].b;
/* Early on the Guest hasn't set the IDT entries (or maybe it put a
* bogus one in): if we fail here, the Guest will be killed. */
if (!idt_present(lo, hi))
return 0;
set_guest_interrupt(lg, lo, hi, has_err(num));
return 1;
}
/*H:250 Here's the hard part: returning to the Host every time a trap happens
* and then calling deliver_trap() and re-entering the Guest is slow.
* Particularly because Guest userspace system calls are traps (trap 128).
*
* So we'd like to set up the IDT to tell the CPU to deliver traps directly
* into the Guest. This is possible, but the complexities cause the size of
* this file to double! However, 150 lines of code is worth writing for taking
* system calls down from 1750ns to 270ns. Plus, if lguest didn't do it, all
* the other hypervisors would tease it.
*
* This routine determines if a trap can be delivered directly. */
static int direct_trap(const struct lguest *lg,
const struct desc_struct *trap,
unsigned int num)
{
/* Hardware interrupts don't go to guest (except syscall). */
/* Hardware interrupts don't go to the Guest at all (except system
* call). */
if (num >= FIRST_EXTERNAL_VECTOR && num != SYSCALL_VECTOR)
return 0;
/* We intercept page fault (demand shadow paging & cr2 saving)
protection fault (in/out emulation) and device not
available (TS handling), and hypercall */
/* The Host needs to see page faults (for shadow paging and to save the
* fault address), general protection faults (in/out emulation) and
* device not available (TS handling), and of course, the hypercall
* trap. */
if (num == 14 || num == 13 || num == 7 || num == LGUEST_TRAP_ENTRY)
return 0;
/* Interrupt gates (0xE) or not present (0x0) can't go direct. */
/* Only trap gates (type 15) can go direct to the Guest. Interrupt
* gates (type 14) disable interrupts as they are entered, which we
* never let the Guest do. Not present entries (type 0x0) also can't
* go direct, of course 8) */
return idt_type(trap->a, trap->b) == 0xF;
}
/*H:260 When we make traps go directly into the Guest, we need to make sure
* the kernel stack is valid (ie. mapped in the page tables). Otherwise, the
* CPU trying to deliver the trap will fault while trying to push the interrupt
* words on the stack: this is called a double fault, and it forces us to kill
* the Guest.
*
* Which is deeply unfair, because (literally!) it wasn't the Guests' fault. */
void pin_stack_pages(struct lguest *lg)
{
unsigned int i;
/* Depending on the CONFIG_4KSTACKS option, the Guest can have one or
* two pages of stack space. */
for (i = 0; i < lg->stack_pages; i++)
/* The stack grows *upwards*, hence the subtraction */
pin_page(lg, lg->esp1 - i * PAGE_SIZE);
}
/* Direct traps also mean that we need to know whenever the Guest wants to use
* a different kernel stack, so we can change the IDT entries to use that
* stack. The IDT entries expect a virtual address, so unlike most addresses
* the Guest gives us, the "esp" (stack pointer) value here is virtual, not
* physical.
*
* In Linux each process has its own kernel stack, so this happens a lot: we
* change stacks on each context switch. */
void guest_set_stack(struct lguest *lg, u32 seg, u32 esp, unsigned int pages)
{
/* You cannot have a stack segment with priv level 0. */
/* You are not allowd have a stack segment with privilege level 0: bad
* Guest! */
if ((seg & 0x3) != GUEST_PL)
kill_guest(lg, "bad stack segment %i", seg);
/* We only expect one or two stack pages. */
if (pages > 2)
kill_guest(lg, "bad stack pages %u", pages);
/* Save where the stack is, and how many pages */
lg->ss1 = seg;
lg->esp1 = esp;
lg->stack_pages = pages;
/* Make sure the new stack pages are mapped */
pin_stack_pages(lg);
}
/* Set up trap in IDT. */
/* All this reference to mapping stacks leads us neatly into the other complex
* part of the Host: page table handling. */
/*H:235 This is the routine which actually checks the Guest's IDT entry and
* transfers it into our entry in "struct lguest": */
static void set_trap(struct lguest *lg, struct desc_struct *trap,
unsigned int num, u32 lo, u32 hi)
{
u8 type = idt_type(lo, hi);
/* We zero-out a not-present entry */
if (!idt_present(lo, hi)) {
trap->a = trap->b = 0;
return;
}
/* We only support interrupt and trap gates. */
if (type != 0xE && type != 0xF)
kill_guest(lg, "bad IDT type %i", type);
/* We only copy the handler address, present bit, privilege level and
* type. The privilege level controls where the trap can be triggered
* manually with an "int" instruction. This is usually GUEST_PL,
* except for system calls which userspace can use. */
trap->a = ((__KERNEL_CS|GUEST_PL)<<16) | (lo&0x0000FFFF);
trap->b = (hi&0xFFFFEF00);
}
/*H:230 While we're here, dealing with delivering traps and interrupts to the
* Guest, we might as well complete the picture: how the Guest tells us where
* it wants them to go. This would be simple, except making traps fast
* requires some tricks.
*
* We saw the Guest setting Interrupt Descriptor Table (IDT) entries with the
* LHCALL_LOAD_IDT_ENTRY hypercall before: that comes here. */
void load_guest_idt_entry(struct lguest *lg, unsigned int num, u32 lo, u32 hi)
{
/* Guest never handles: NMI, doublefault, hypercall, spurious irq. */
/* Guest never handles: NMI, doublefault, spurious interrupt or
* hypercall. We ignore when it tries to set them. */
if (num == 2 || num == 8 || num == 15 || num == LGUEST_TRAP_ENTRY)
return;
/* Mark the IDT as changed: next time the Guest runs we'll know we have
* to copy this again. */
lg->changed |= CHANGED_IDT;
/* The IDT which we keep in "struct lguest" only contains 32 entries
* for the traps and LGUEST_IRQS (32) entries for interrupts. We
* ignore attempts to set handlers for higher interrupt numbers, except
* for the system call "interrupt" at 128: we have a special IDT entry
* for that. */
if (num < ARRAY_SIZE(lg->idt))
set_trap(lg, &lg->idt[num], num, lo, hi);
else if (num == SYSCALL_VECTOR)
set_trap(lg, &lg->syscall_idt, num, lo, hi);
}
/* The default entry for each interrupt points into the Switcher routines which
* simply return to the Host. The run_guest() loop will then call
* deliver_trap() to bounce it back into the Guest. */
static void default_idt_entry(struct desc_struct *idt,
int trap,
const unsigned long handler)
{
/* A present interrupt gate. */
u32 flags = 0x8e00;
/* They can't "int" into any of them except hypercall. */
/* Set the privilege level on the entry for the hypercall: this allows
* the Guest to use the "int" instruction to trigger it. */
if (trap == LGUEST_TRAP_ENTRY)
flags |= (GUEST_PL << 13);
/* Now pack it into the IDT entry in its weird format. */
idt->a = (LGUEST_CS<<16) | (handler&0x0000FFFF);
idt->b = (handler&0xFFFF0000) | flags;
}
/* When the Guest first starts, we put default entries into the IDT. */
void setup_default_idt_entries(struct lguest_ro_state *state,
const unsigned long *def)
{
@ -230,19 +362,25 @@ void setup_default_idt_entries(struct lguest_ro_state *state,
default_idt_entry(&state->guest_idt[i], i, def[i]);
}
/*H:240 We don't use the IDT entries in the "struct lguest" directly, instead
* we copy them into the IDT which we've set up for Guests on this CPU, just
* before we run the Guest. This routine does that copy. */
void copy_traps(const struct lguest *lg, struct desc_struct *idt,
const unsigned long *def)
{
unsigned int i;
/* All hardware interrupts are same whatever the guest: only the
* traps might be different. */
/* We can simply copy the direct traps, otherwise we use the default
* ones in the Switcher: they will return to the Host. */
for (i = 0; i < FIRST_EXTERNAL_VECTOR; i++) {
if (direct_trap(lg, &lg->idt[i], i))
idt[i] = lg->idt[i];
else
default_idt_entry(&idt[i], i, def[i]);
}
/* Don't forget the system call trap! The IDT entries for other
* interupts never change, so no need to copy them. */
i = SYSCALL_VECTOR;
if (direct_trap(lg, &lg->syscall_idt, i))
idt[i] = lg->syscall_idt;

19
drivers/lguest/lg.h
View File

@ -58,9 +58,18 @@ struct lguest_dma_info
u8 interrupt; /* 0 when not registered */
};
/* We have separate types for the guest's ptes & pgds and the shadow ptes &
* pgds. Since this host might use three-level pagetables and the guest and
* shadow pagetables don't, we can't use the normal pte_t/pgd_t. */
/*H:310 The page-table code owes a great debt of gratitude to Andi Kleen. He
* reviewed the original code which used "u32" for all page table entries, and
* insisted that it would be far clearer with explicit typing. I thought it
* was overkill, but he was right: it is much clearer than it was before.
*
* We have separate types for the Guest's ptes & pgds and the shadow ptes &
* pgds. There's already a Linux type for these (pte_t and pgd_t) but they
* change depending on kernel config options (PAE). */
/* Each entry is identical: lower 12 bits of flags and upper 20 bits for the
* "page frame number" (0 == first physical page, etc). They are different
* types so the compiler will warn us if we mix them improperly. */
typedef union {
struct { unsigned flags:12, pfn:20; };
struct { unsigned long val; } raw;
@ -77,8 +86,12 @@ typedef union {
struct { unsigned flags:12, pfn:20; };
struct { unsigned long val; } raw;
} gpte_t;
/* We have two convenient macros to convert a "raw" value as handed to us by
* the Guest into the correct Guest PGD or PTE type. */
#define mkgpte(_val) ((gpte_t){.raw.val = _val})
#define mkgpgd(_val) ((gpgd_t){.raw.val = _val})
/*:*/
struct pgdir
{

314
drivers/lguest/page_tables.c
View File

@ -15,38 +15,91 @@
#include <asm/tlbflush.h>
#include "lg.h"
/*H:300
* The Page Table Code
*
* We use two-level page tables for the Guest. If you're not entirely
* comfortable with virtual addresses, physical addresses and page tables then
* I recommend you review lguest.c's "Page Table Handling" (with diagrams!).
*
* The Guest keeps page tables, but we maintain the actual ones here: these are
* called "shadow" page tables. Which is a very Guest-centric name: these are
* the real page tables the CPU uses, although we keep them up to date to
* reflect the Guest's. (See what I mean about weird naming? Since when do
* shadows reflect anything?)
*
* Anyway, this is the most complicated part of the Host code. There are seven
* parts to this:
* (i) Setting up a page table entry for the Guest when it faults,
* (ii) Setting up the page table entry for the Guest stack,
* (iii) Setting up a page table entry when the Guest tells us it has changed,
* (iv) Switching page tables,
* (v) Flushing (thowing away) page tables,
* (vi) Mapping the Switcher when the Guest is about to run,
* (vii) Setting up the page tables initially.
:*/
/* Pages a 4k long, and each page table entry is 4 bytes long, giving us 1024
* (or 2^10) entries per page. */
#define PTES_PER_PAGE_SHIFT 10
#define PTES_PER_PAGE (1 << PTES_PER_PAGE_SHIFT)
/* 1024 entries in a page table page maps 1024 pages: 4MB. The Switcher is
* conveniently placed at the top 4MB, so it uses a separate, complete PTE
* page. */
#define SWITCHER_PGD_INDEX (PTES_PER_PAGE - 1)
/* We actually need a separate PTE page for each CPU. Remember that after the
* Switcher code itself comes two pages for each CPU, and we don't want this
* CPU's guest to see the pages of any other CPU. */
static DEFINE_PER_CPU(spte_t *, switcher_pte_pages);
#define switcher_pte_page(cpu) per_cpu(switcher_pte_pages, cpu)
/*H:320 With our shadow and Guest types established, we need to deal with
* them: the page table code is curly enough to need helper functions to keep
* it clear and clean.
*
* The first helper takes a virtual address, and says which entry in the top
* level page table deals with that address. Since each top level entry deals
* with 4M, this effectively divides by 4M. */
static unsigned vaddr_to_pgd_index(unsigned long vaddr)
{
return vaddr >> (PAGE_SHIFT + PTES_PER_PAGE_SHIFT);
}
/* These access the shadow versions (ie. the ones used by the CPU). */
/* There are two functions which return pointers to the shadow (aka "real")
* page tables.
*
* spgd_addr() takes the virtual address and returns a pointer to the top-level
* page directory entry for that address. Since we keep track of several page
* tables, the "i" argument tells us which one we're interested in (it's
* usually the current one). */
static spgd_t *spgd_addr(struct lguest *lg, u32 i, unsigned long vaddr)
{
unsigned int index = vaddr_to_pgd_index(vaddr);
/* We kill any Guest trying to touch the Switcher addresses. */
if (index >= SWITCHER_PGD_INDEX) {
kill_guest(lg, "attempt to access switcher pages");
index = 0;
}
/* Return a pointer index'th pgd entry for the i'th page table. */
return &lg->pgdirs[i].pgdir[index];
}
/* This routine then takes the PGD entry given above, which contains the
* address of the PTE page. It then returns a pointer to the PTE entry for the
* given address. */
static spte_t *spte_addr(struct lguest *lg, spgd_t spgd, unsigned long vaddr)
{
spte_t *page = __va(spgd.pfn << PAGE_SHIFT);
/* You should never call this if the PGD entry wasn't valid */
BUG_ON(!(spgd.flags & _PAGE_PRESENT));
return &page[(vaddr >> PAGE_SHIFT) % PTES_PER_PAGE];
}
/* These access the guest versions. */
/* These two functions just like the above two, except they access the Guest
* page tables. Hence they return a Guest address. */
static unsigned long gpgd_addr(struct lguest *lg, unsigned long vaddr)
{
unsigned int index = vaddr >> (PAGE_SHIFT + PTES_PER_PAGE_SHIFT);
@ -61,12 +114,24 @@ static unsigned long gpte_addr(struct lguest *lg,
return gpage + ((vaddr>>PAGE_SHIFT) % PTES_PER_PAGE) * sizeof(gpte_t);
}
/* Do a virtual -> physical mapping on a user page. */
/*H:350 This routine takes a page number given by the Guest and converts it to
* an actual, physical page number. It can fail for several reasons: the
* virtual address might not be mapped by the Launcher, the write flag is set
* and the page is read-only, or the write flag was set and the page was
* shared so had to be copied, but we ran out of memory.
*
* This holds a reference to the page, so release_pte() is careful to
* put that back. */
static unsigned long get_pfn(unsigned long virtpfn, int write)
{
struct page *page;
/* This value indicates failure. */
unsigned long ret = -1UL;
/* get_user_pages() is a complex interface: it gets the "struct
* vm_area_struct" and "struct page" assocated with a range of pages.
* It also needs the task's mmap_sem held, and is not very quick.
* It returns the number of pages it got. */
down_read(&current->mm->mmap_sem);
if (get_user_pages(current, current->mm, virtpfn << PAGE_SHIFT,
1, write, 1, &page, NULL) == 1)
@ -75,28 +140,47 @@ static unsigned long get_pfn(unsigned long virtpfn, int write)
return ret;
}
/*H:340 Converting a Guest page table entry to a shadow (ie. real) page table
* entry can be a little tricky. The flags are (almost) the same, but the
* Guest PTE contains a virtual page number: the CPU needs the real page
* number. */
static spte_t gpte_to_spte(struct lguest *lg, gpte_t gpte, int write)
{
spte_t spte;
unsigned long pfn;
/* We ignore the global flag. */
/* The Guest sets the global flag, because it thinks that it is using
* PGE. We only told it to use PGE so it would tell us whether it was
* flushing a kernel mapping or a userspace mapping. We don't actually
* use the global bit, so throw it away. */
spte.flags = (gpte.flags & ~_PAGE_GLOBAL);
/* We need a temporary "unsigned long" variable to hold the answer from
* get_pfn(), because it returns 0xFFFFFFFF on failure, which wouldn't
* fit in spte.pfn. get_pfn() finds the real physical number of the
* page, given the virtual number. */
pfn = get_pfn(gpte.pfn, write);
if (pfn == -1UL) {
kill_guest(lg, "failed to get page %u", gpte.pfn);
/* Must not put_page() bogus page on cleanup. */
/* When we destroy the Guest, we'll go through the shadow page
* tables and release_pte() them. Make sure we don't think
* this one is valid! */
spte.flags = 0;
}
/* Now we assign the page number, and our shadow PTE is complete. */
spte.pfn = pfn;
return spte;
}
/*H:460 And to complete the chain, release_pte() looks like this: */
static void release_pte(spte_t pte)
{
/* Remember that get_user_pages() took a reference to the page, in
* get_pfn()? We have to put it back now. */
if (pte.flags & _PAGE_PRESENT)
put_page(pfn_to_page(pte.pfn));
}
/*:*/
static void check_gpte(struct lguest *lg, gpte_t gpte)
{
@ -110,11 +194,16 @@ static void check_gpgd(struct lguest *lg, gpgd_t gpgd)
kill_guest(lg, "bad page directory entry");
}
/* FIXME: We hold reference to pages, which prevents them from being
swapped. It'd be nice to have a callback when Linux wants to swap out. */
/* We fault pages in, which allows us to update accessed/dirty bits.
* Return true if we got page. */
/*H:330
* (i) Setting up a page table entry for the Guest when it faults
*
* We saw this call in run_guest(): when we see a page fault in the Guest, we
* come here. That's because we only set up the shadow page tables lazily as
* they're needed, so we get page faults all the time and quietly fix them up
* and return to the Guest without it knowing.
*
* If we fixed up the fault (ie. we mapped the address), this routine returns
* true. */
int demand_page(struct lguest *lg, unsigned long vaddr, int errcode)
{
gpgd_t gpgd;
@ -123,106 +212,161 @@ int demand_page(struct lguest *lg, unsigned long vaddr, int errcode)
gpte_t gpte;
spte_t *spte;
/* First step: get the top-level Guest page table entry. */
gpgd = mkgpgd(lgread_u32(lg, gpgd_addr(lg, vaddr)));
/* Toplevel not present? We can't map it in. */
if (!(gpgd.flags & _PAGE_PRESENT))
return 0;
/* Now look at the matching shadow entry. */
spgd = spgd_addr(lg, lg->pgdidx, vaddr);
if (!(spgd->flags & _PAGE_PRESENT)) {
/* Get a page of PTEs for them. */
/* No shadow entry: allocate a new shadow PTE page. */
unsigned long ptepage = get_zeroed_page(GFP_KERNEL);
/* FIXME: Steal from self in this case? */
/* This is not really the Guest's fault, but killing it is
* simple for this corner case. */
if (!ptepage) {
kill_guest(lg, "out of memory allocating pte page");
return 0;
}
/* We check that the Guest pgd is OK. */
check_gpgd(lg, gpgd);
/* And we copy the flags to the shadow PGD entry. The page
* number in the shadow PGD is the page we just allocated. */
spgd->raw.val = (__pa(ptepage) | gpgd.flags);
}
/* OK, now we look at the lower level in the Guest page table: keep its
* address, because we might update it later. */
gpte_ptr = gpte_addr(lg, gpgd, vaddr);
gpte = mkgpte(lgread_u32(lg, gpte_ptr));
/* No page? */
/* If this page isn't in the Guest page tables, we can't page it in. */
if (!(gpte.flags & _PAGE_PRESENT))
return 0;
/* Write to read-only page? */
/* Check they're not trying to write to a page the Guest wants
* read-only (bit 2 of errcode == write). */
if ((errcode & 2) && !(gpte.flags & _PAGE_RW))
return 0;
/* User access to a non-user page? */
/* User access to a kernel page? (bit 3 == user access) */
if ((errcode & 4) && !(gpte.flags & _PAGE_USER))
return 0;
/* Check that the Guest PTE flags are OK, and the page number is below
* the pfn_limit (ie. not mapping the Launcher binary). */
check_gpte(lg, gpte);
/* Add the _PAGE_ACCESSED and (for a write) _PAGE_DIRTY flag */
gpte.flags |= _PAGE_ACCESSED;
if (errcode & 2)
gpte.flags |= _PAGE_DIRTY;
/* We're done with the old pte. */
/* Get the pointer to the shadow PTE entry we're going to set. */
spte = spte_addr(lg, *spgd, vaddr);
/* If there was a valid shadow PTE entry here before, we release it.
* This can happen with a write to a previously read-only entry. */
release_pte(*spte);
/* We don't make it writable if this isn't a write: later
* write will fault so we can set dirty bit in guest. */
/* If this is a write, we insist that the Guest page is writable (the
* final arg to gpte_to_spte()). */
if (gpte.flags & _PAGE_DIRTY)
*spte = gpte_to_spte(lg, gpte, 1);
else {
/* If this is a read, don't set the "writable" bit in the page
* table entry, even if the Guest says it's writable. That way
* we come back here when a write does actually ocur, so we can
* update the Guest's _PAGE_DIRTY flag. */
gpte_t ro_gpte = gpte;
ro_gpte.flags &= ~_PAGE_RW;
*spte = gpte_to_spte(lg, ro_gpte, 0);
}
/* Now we update dirty/accessed on guest. */
/* Finally, we write the Guest PTE entry back: we've set the
* _PAGE_ACCESSED and maybe the _PAGE_DIRTY flags. */
lgwrite_u32(lg, gpte_ptr, gpte.raw.val);
/* We succeeded in mapping the page! */
return 1;
}
/* This is much faster than the full demand_page logic. */
/*H:360 (ii) Setting up the page table entry for the Guest stack.
*
* Remember pin_stack_pages() which makes sure the stack is mapped? It could
* simply call demand_page(), but as we've seen that logic is quite long, and
* usually the stack pages are already mapped anyway, so it's not required.
*
* This is a quick version which answers the question: is this virtual address
* mapped by the shadow page tables, and is it writable? */
static int page_writable(struct lguest *lg, unsigned long vaddr)
{
spgd_t *spgd;
unsigned long flags;
/* Look at the top level entry: is it present? */
spgd = spgd_addr(lg, lg->pgdidx, vaddr);
if (!(spgd->flags & _PAGE_PRESENT))
return 0;
/* Check the flags on the pte entry itself: it must be present and
* writable. */
flags = spte_addr(lg, *spgd, vaddr)->flags;
return (flags & (_PAGE_PRESENT|_PAGE_RW)) == (_PAGE_PRESENT|_PAGE_RW);
}
/* So, when pin_stack_pages() asks us to pin a page, we check if it's already
* in the page tables, and if not, we call demand_page() with error code 2
* (meaning "write"). */
void pin_page(struct lguest *lg, unsigned long vaddr)
{
if (!page_writable(lg, vaddr) && !demand_page(lg, vaddr, 2))
kill_guest(lg, "bad stack page %#lx", vaddr);
}
/*H:450 If we chase down the release_pgd() code, it looks like this: */
static void release_pgd(struct lguest *lg, spgd_t *spgd)
{
/* If the entry's not present, there's nothing to release. */
if (spgd->flags & _PAGE_PRESENT) {
unsigned int i;
/* Converting the pfn to find the actual PTE page is easy: turn
* the page number into a physical address, then convert to a
* virtual address (easy for kernel pages like this one). */
spte_t *ptepage = __va(spgd->pfn << PAGE_SHIFT);
/* For each entry in the page, we might need to release it. */
for (i = 0; i < PTES_PER_PAGE; i++)
release_pte(ptepage[i]);
/* Now we can free the page of PTEs */
free_page((long)ptepage);
/* And zero out the PGD entry we we never release it twice. */
spgd->raw.val = 0;
}
}
/*H:440 (v) Flushing (thowing away) page tables,
*
* We saw flush_user_mappings() called when we re-used a top-level pgdir page.
* It simply releases every PTE page from 0 up to the kernel address. */
static void flush_user_mappings(struct lguest *lg, int idx)
{
unsigned int i;
/* Release every pgd entry up to the kernel's address. */
for (i = 0; i < vaddr_to_pgd_index(lg->page_offset); i++)
release_pgd(lg, lg->pgdirs[idx].pgdir + i);
}
/* The Guest also has a hypercall to do this manually: it's used when a large
* number of mappings have been changed. */
void guest_pagetable_flush_user(struct lguest *lg)
{
/* Drop the userspace part of the current page table. */
flush_user_mappings(lg, lg->pgdidx);
}
/*:*/
/* We keep several page tables. This is a simple routine to find the page
* table (if any) corresponding to this top-level address the Guest has given
* us. */
static unsigned int find_pgdir(struct lguest *lg, unsigned long pgtable)
{
unsigned int i;
@ -232,21 +376,30 @@ static unsigned int find_pgdir(struct lguest *lg, unsigned long pgtable)
return i;
}
/*H:435 And this is us, creating the new page directory. If we really do
* allocate a new one (and so the kernel parts are not there), we set
* blank_pgdir. */
static unsigned int new_pgdir(struct lguest *lg,
unsigned long cr3,
int *blank_pgdir)
{
unsigned int next;
/* We pick one entry at random to throw out. Choosing the Least
* Recently Used might be better, but this is easy. */
next = random32() % ARRAY_SIZE(lg->pgdirs);
/* If it's never been allocated at all before, try now. */
if (!lg->pgdirs[next].pgdir) {
lg->pgdirs[next].pgdir = (spgd_t *)get_zeroed_page(GFP_KERNEL);
/* If the allocation fails, just keep using the one we have */
if (!lg->pgdirs[next].pgdir)
next = lg->pgdidx;
else
/* There are no mappings: you'll need to re-pin */
/* This is a blank page, so there are no kernel
* mappings: caller must map the stack! */
*blank_pgdir = 1;
}
/* Record which Guest toplevel this shadows. */
lg->pgdirs[next].cr3 = cr3;
/* Release all the non-kernel mappings. */
flush_user_mappings(lg, next);
@ -254,82 +407,161 @@ static unsigned int new_pgdir(struct lguest *lg,
return next;
}
/*H:430 (iv) Switching page tables
*
* This is what happens when the Guest changes page tables (ie. changes the
* top-level pgdir). This happens on almost every context switch. */
void guest_new_pagetable(struct lguest *lg, unsigned long pgtable)
{
int newpgdir, repin = 0;
/* Look to see if we have this one already. */
newpgdir = find_pgdir(lg, pgtable);
/* If not, we allocate or mug an existing one: if it's a fresh one,
* repin gets set to 1. */
if (newpgdir == ARRAY_SIZE(lg->pgdirs))
newpgdir = new_pgdir(lg, pgtable, &repin);
/* Change the current pgd index to the new one. */
lg->pgdidx = newpgdir;
/* If it was completely blank, we map in the Guest kernel stack */
if (repin)
pin_stack_pages(lg);
}
/*H:470 Finally, a routine which throws away everything: all PGD entries in all
* the shadow page tables. This is used when we destroy the Guest. */
static void release_all_pagetables(struct lguest *lg)
{
unsigned int i, j;
/* Every shadow pagetable this Guest has */
for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++)
if (lg->pgdirs[i].pgdir)
/* Every PGD entry except the Switcher at the top */
for (j = 0; j < SWITCHER_PGD_INDEX; j++)
release_pgd(lg, lg->pgdirs[i].pgdir + j);
}
/* We also throw away everything when a Guest tells us it's changed a kernel
* mapping. Since kernel mappings are in every page table, it's easiest to
* throw them all away. This is amazingly slow, but thankfully rare. */
void guest_pagetable_clear_all(struct lguest *lg)
{
release_all_pagetables(lg);
/* We need the Guest kernel stack mapped again. */
pin_stack_pages(lg);
}
/*H:420 This is the routine which actually sets the page table entry for then
* "idx"'th shadow page table.
*
* Normally, we can just throw out the old entry and replace it with 0: if they
* use it demand_page() will put the new entry in. We need to do this anyway:
* The Guest expects _PAGE_ACCESSED to be set on its PTE the first time a page
* is read from, and _PAGE_DIRTY when it's written to.
*
* But Avi Kivity pointed out that most Operating Systems (Linux included) set
* these bits on PTEs immediately anyway. This is done to save the CPU from
* having to update them, but it helps us the same way: if they set
* _PAGE_ACCESSED then we can put a read-only PTE entry in immediately, and if
* they set _PAGE_DIRTY then we can put a writable PTE entry in immediately.
*/
static void do_set_pte(struct lguest *lg, int idx,
unsigned long vaddr, gpte_t gpte)
{
/* Look up the matching shadow page directot entry. */
spgd_t *spgd = spgd_addr(lg, idx, vaddr);
/* If the top level isn't present, there's no entry to update. */
if (spgd->flags & _PAGE_PRESENT) {
/* Otherwise, we start by releasing the existing entry. */
spte_t *spte = spte_addr(lg, *spgd, vaddr);
release_pte(*spte);
/* If they're setting this entry as dirty or accessed, we might
* as well put that entry they've given us in now. This shaves
* 10% off a copy-on-write micro-benchmark. */
if (gpte.flags & (_PAGE_DIRTY | _PAGE_ACCESSED)) {
check_gpte(lg, gpte);
*spte = gpte_to_spte(lg, gpte, gpte.flags&_PAGE_DIRTY);
} else
/* Otherwise we can demand_page() it in later. */
spte->raw.val = 0;
}
}
/*H:410 Updating a PTE entry is a little trickier.
*
* We keep track of several different page tables (the Guest uses one for each
* process, so it makes sense to cache at least a few). Each of these have
* identical kernel parts: ie. every mapping above PAGE_OFFSET is the same for
* all processes. So when the page table above that address changes, we update
* all the page tables, not just the current one. This is rare.
*
* The benefit is that when we have to track a new page table, we can copy keep
* all the kernel mappings. This speeds up context switch immensely. */
void guest_set_pte(struct lguest *lg,
unsigned long cr3, unsigned long vaddr, gpte_t gpte)
{
/* Kernel mappings must be changed on all top levels. */
/* Kernel mappings must be changed on all top levels. Slow, but
* doesn't happen often. */
if (vaddr >= lg->page_offset) {
unsigned int i;
for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++)
if (lg->pgdirs[i].pgdir)
do_set_pte(lg, i, vaddr, gpte);
} else {
/* Is this page table one we have a shadow for? */
int pgdir = find_pgdir(lg, cr3);
if (pgdir != ARRAY_SIZE(lg->pgdirs))
/* If so, do the update. */
do_set_pte(lg, pgdir, vaddr, gpte);
}
}
/*H:400
* (iii) Setting up a page table entry when the Guest tells us it has changed.
*
* Just like we did in interrupts_and_traps.c, it makes sense for us to deal
* with the other side of page tables while we're here: what happens when the
* Guest asks for a page table to be updated?
*
* We already saw that demand_page() will fill in the shadow page tables when
* needed, so we can simply remove shadow page table entries whenever the Guest
* tells us they've changed. When the Guest tries to use the new entry it will
* fault and demand_page() will fix it up.
*
* So with that in mind here's our code to to update a (top-level) PGD entry:
*/
void guest_set_pmd(struct lguest *lg, unsigned long cr3, u32 idx)
{
int pgdir;
/* The kernel seems to try to initialize this early on: we ignore its
* attempts to map over the Switcher. */
if (idx >= SWITCHER_PGD_INDEX)
return;
/* If they're talking about a page table we have a shadow for... */
pgdir = find_pgdir(lg, cr3);
if (pgdir < ARRAY_SIZE(lg->pgdirs))
/* ... throw it away. */
release_pgd(lg, lg->pgdirs[pgdir].pgdir + idx);
}
/*H:500 (vii) Setting up the page tables initially.
*
* When a Guest is first created, the Launcher tells us where the toplevel of
* its first page table is. We set some things up here: */
int init_guest_pagetable(struct lguest *lg, unsigned long pgtable)
{
/* We assume this in flush_user_mappings, so check now */
/* In flush_user_mappings() we loop from 0 to
* "vaddr_to_pgd_index(lg->page_offset)". This assumes it won't hit
* the Switcher mappings, so check that now. */
if (vaddr_to_pgd_index(lg->page_offset) >= SWITCHER_PGD_INDEX)
return -EINVAL;
/* We start on the first shadow page table, and give it a blank PGD
* page. */
lg->pgdidx = 0;
lg->pgdirs[lg->pgdidx].cr3 = pgtable;
lg->pgdirs[lg->pgdidx].pgdir = (spgd_t*)get_zeroed_page(GFP_KERNEL);
@ -338,33 +570,48 @@ int init_guest_pagetable(struct lguest *lg, unsigned long pgtable)
return 0;
}
/* When a Guest dies, our cleanup is fairly simple. */
void free_guest_pagetable(struct lguest *lg)
{
unsigned int i;
/* Throw away all page table pages. */
release_all_pagetables(lg);
/* Now free the top levels: free_page() can handle 0 just fine. */
for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++)
free_page((long)lg->pgdirs[i].pgdir);
}
/* Caller must be preempt-safe */
/*H:480 (vi) Mapping the Switcher when the Guest is about to run.
*
* The Switcher and the two pages for this CPU need to be available to the
* Guest (and not the pages for other CPUs). We have the appropriate PTE pages
* for each CPU already set up, we just need to hook them in. */
void map_switcher_in_guest(struct lguest *lg, struct lguest_pages *pages)
{
spte_t *switcher_pte_page = __get_cpu_var(switcher_pte_pages);
spgd_t switcher_pgd;
spte_t regs_pte;
/* Since switcher less that 4MB, we simply mug top pte page. */
/* Make the last PGD entry for this Guest point to the Switcher's PTE
* page for this CPU (with appropriate flags). */
switcher_pgd.pfn = __pa(switcher_pte_page) >> PAGE_SHIFT;
switcher_pgd.flags = _PAGE_KERNEL;
lg->pgdirs[lg->pgdidx].pgdir[SWITCHER_PGD_INDEX] = switcher_pgd;
/* Map our regs page over stack page. */
/* We also change the Switcher PTE page. When we're running the Guest,
* we want the Guest's "regs" page to appear where the first Switcher
* page for this CPU is. This is an optimization: when the Switcher
* saves the Guest registers, it saves them into the first page of this
* CPU's "struct lguest_pages": if we make sure the Guest's register
* page is already mapped there, we don't have to copy them out
* again. */
regs_pte.pfn = __pa(lg->regs_page) >> PAGE_SHIFT;
regs_pte.flags = _PAGE_KERNEL;
switcher_pte_page[(unsigned long)pages/PAGE_SIZE%PTES_PER_PAGE]
= regs_pte;
}
/*:*/
static void free_switcher_pte_pages(void)
{
@ -374,6 +621,10 @@ static void free_switcher_pte_pages(void)
free_page((long)switcher_pte_page(i));
}
/*H:520 Setting up the Switcher PTE page for given CPU is fairly easy, given
* the CPU number and the "struct page"s for the Switcher code itself.
*
* Currently the Switcher is less than a page long, so "pages" is always 1. */
static __init void populate_switcher_pte_page(unsigned int cpu,
struct page *switcher_page[],
unsigned int pages)
@ -381,21 +632,26 @@ static __init void populate_switcher_pte_page(unsigned int cpu,
unsigned int i;
spte_t *pte = switcher_pte_page(cpu);
/* The first entries are easy: they map the Switcher code. */
for (i = 0; i < pages; i++) {
pte[i].pfn = page_to_pfn(switcher_page[i]);
pte[i].flags = _PAGE_PRESENT|_PAGE_ACCESSED;
}
/* We only map this CPU's pages, so guest can't see others. */
/* The only other thing we map is this CPU's pair of pages. */
i = pages + cpu*2;
/* First page (regs) is rw, second (state) is ro. */
/* First page (Guest registers) is writable from the Guest */
pte[i].pfn = page_to_pfn(switcher_page[i]);
pte[i].flags = _PAGE_PRESENT|_PAGE_ACCESSED|_PAGE_RW;
/* The second page contains the "struct lguest_ro_state", and is
* read-only. */
pte[i+1].pfn = page_to_pfn(switcher_page[i+1]);
pte[i+1].flags = _PAGE_PRESENT|_PAGE_ACCESSED;
}
/*H:510 At boot or module load time, init_pagetables() allocates and populates
* the Switcher PTE page for each CPU. */
__init int init_pagetables(struct page **switcher_page, unsigned int pages)
{
unsigned int i;
@ -410,7 +666,9 @@ __init int init_pagetables(struct page **switcher_page, unsigned int pages)
}
return 0;
}
/*:*/
/* Cleaning up simply involves freeing the PTE page for each CPU. */
void free_pagetables(void)
{
free_switcher_pte_pages();

109
drivers/lguest/segments.c
View File

@ -11,17 +11,58 @@
* from frolicking through its parklike serenity. :*/
#include "lg.h"
/*H:600
* We've almost completed the Host; there's just one file to go!
*
* Segments & The Global Descriptor Table
*
* (That title sounds like a bad Nerdcore group. Not to suggest that there are
* any good Nerdcore groups, but in high school a friend of mine had a band
* called Joe Fish and the Chips, so there are definitely worse band names).
*
* To refresh: the GDT is a table of 8-byte values describing segments. Once
* set up, these segments can be loaded into one of the 6 "segment registers".
*
* GDT entries are passed around as "struct desc_struct"s, which like IDT
* entries are split into two 32-bit members, "a" and "b". One day, someone
* will clean that up, and be declared a Hero. (No pressure, I'm just saying).
*
* Anyway, the GDT entry contains a base (the start address of the segment), a
* limit (the size of the segment - 1), and some flags. Sounds simple, and it
* would be, except those zany Intel engineers decided that it was too boring
* to put the base at one end, the limit at the other, and the flags in
* between. They decided to shotgun the bits at random throughout the 8 bytes,
* like so:
*
* 0 16 40 48 52 56 63
* [ limit part 1 ][ base part 1 ][ flags ][li][fl][base ]
* mit ags part 2
* part 2
*
* As a result, this file contains a certain amount of magic numeracy. Let's
* begin.
*/
/* Is the descriptor the Guest wants us to put in OK?
*
* The flag which Intel says must be zero: must be zero. The descriptor must
* be present, (this is actually checked earlier but is here for thorougness),
* and the descriptor type must be 1 (a memory segment). */
static int desc_ok(const struct desc_struct *gdt)
{
/* MBZ=0, P=1, DT=1 */
return ((gdt->b & 0x00209000) == 0x00009000);
}
/* Is the segment present? (Otherwise it can't be used by the Guest). */
static int segment_present(const struct desc_struct *gdt)
{
return gdt->b & 0x8000;
}
/* There are several entries we don't let the Guest set. The TSS entry is the
* "Task State Segment" which controls all kinds of delicate things. The
* LGUEST_CS and LGUEST_DS entries are reserved for the Switcher, and the
* the Guest can't be trusted to deal with double faults. */
static int ignored_gdt(unsigned int num)
{
return (num == GDT_ENTRY_TSS
@ -30,9 +71,18 @@ static int ignored_gdt(unsigned int num)
|| num == GDT_ENTRY_DOUBLEFAULT_TSS);
}
/* We don't allow removal of CS, DS or SS; it doesn't make sense. */
/* If the Guest asks us to remove an entry from the GDT, we have to be careful.
* If one of the segment registers is pointing at that entry the Switcher will
* crash when it tries to reload the segment registers for the Guest.
*
* It doesn't make much sense for the Guest to try to remove its own code, data
* or stack segments while they're in use: assume that's a Guest bug. If it's
* one of the lesser segment registers using the removed entry, we simply set
* that register to 0 (unusable). */
static void check_segment_use(struct lguest *lg, unsigned int desc)
{
/* GDT entries are 8 bytes long, so we divide to get the index and
* ignore the bottom bits. */
if (lg->regs->gs / 8 == desc)
lg->regs->gs = 0;
if (lg->regs->fs / 8 == desc)
@ -45,12 +95,16 @@ static void check_segment_use(struct lguest *lg, unsigned int desc)
kill_guest(lg, "Removed live GDT entry %u", desc);
}
/*H:610 Once the GDT has been changed, we look through the changed entries and
* see if they're OK. If not, we'll call kill_guest() and the Guest will never
* get to use the invalid entries. */
static void fixup_gdt_table(struct lguest *lg, unsigned start, unsigned end)
{
unsigned int i;
for (i = start; i < end; i++) {
/* We never copy these ones to real gdt */
/* We never copy these ones to real GDT, so we don't care what
* they say */
if (ignored_gdt(i))
continue;
@ -64,41 +118,57 @@ static void fixup_gdt_table(struct lguest *lg, unsigned start, unsigned end)
if (!desc_ok(&lg->gdt[i]))
kill_guest(lg, "Bad GDT descriptor %i", i);
/* DPL 0 presumably means "for use by guest". */
/* Segment descriptors contain a privilege level: the Guest is
* sometimes careless and leaves this as 0, even though it's
* running at privilege level 1. If so, we fix it here. */
if ((lg->gdt[i].b & 0x00006000) == 0)
lg->gdt[i].b |= (GUEST_PL << 13);
/* Set accessed bit, since gdt isn't writable. */
/* Each descriptor has an "accessed" bit. If we don't set it
* now, the CPU will try to set it when the Guest first loads
* that entry into a segment register. But the GDT isn't
* writable by the Guest, so bad things can happen. */
lg->gdt[i].b |= 0x00000100;
}
}
/* This routine is called at boot or modprobe time for each CPU to set up the
* "constant" GDT entries for Guests running on that CPU. */
void setup_default_gdt_entries(struct lguest_ro_state *state)
{
struct desc_struct *gdt = state->guest_gdt;
unsigned long tss = (unsigned long)&state->guest_tss;
/* Hypervisor segments. */
/* The hypervisor segments are full 0-4G segments, privilege level 0 */
gdt[GDT_ENTRY_LGUEST_CS] = FULL_EXEC_SEGMENT;
gdt[GDT_ENTRY_LGUEST_DS] = FULL_SEGMENT;
/* This is the one which we *cannot* copy from guest, since tss
is depended on this lguest_ro_state, ie. this cpu. */
/* The TSS segment refers to the TSS entry for this CPU, so we cannot
* copy it from the Guest. Forgive the magic flags */
gdt[GDT_ENTRY_TSS].a = 0x00000067 | (tss << 16);
gdt[GDT_ENTRY_TSS].b = 0x00008900 | (tss & 0xFF000000)
| ((tss >> 16) & 0x000000FF);
}
/* This routine is called before the Guest is run for the first time. */
void setup_guest_gdt(struct lguest *lg)
{
/* Start with full 0-4G segments... */
lg->gdt[GDT_ENTRY_KERNEL_CS] = FULL_EXEC_SEGMENT;
lg->gdt[GDT_ENTRY_KERNEL_DS] = FULL_SEGMENT;
/* ...except the Guest is allowed to use them, so set the privilege
* level appropriately in the flags. */
lg->gdt[GDT_ENTRY_KERNEL_CS].b |= (GUEST_PL << 13);
lg->gdt[GDT_ENTRY_KERNEL_DS].b |= (GUEST_PL << 13);
}
/* This is a fast version for the common case where only the three TLS entries
* have changed. */
/* Like the IDT, we never simply use the GDT the Guest gives us. We set up the
* GDTs for each CPU, then we copy across the entries each time we want to run
* a different Guest on that CPU. */
/* A partial GDT load, for the three "thead-local storage" entries. Otherwise
* it's just like load_guest_gdt(). So much, in fact, it would probably be
* neater to have a single hypercall to cover both. */
void copy_gdt_tls(const struct lguest *lg, struct desc_struct *gdt)
{
unsigned int i;
@ -107,22 +177,31 @@ void copy_gdt_tls(const struct lguest *lg, struct desc_struct *gdt)
gdt[i] = lg->gdt[i];
}
/* This is the full version */
void copy_gdt(const struct lguest *lg, struct desc_struct *gdt)
{
unsigned int i;
/* The default entries from setup_default_gdt_entries() are not
* replaced. See ignored_gdt() above. */
for (i = 0; i < GDT_ENTRIES; i++)
if (!ignored_gdt(i))
gdt[i] = lg->gdt[i];
}
/* This is where the Guest asks us to load a new GDT (LHCALL_LOAD_GDT). */
void load_guest_gdt(struct lguest *lg, unsigned long table, u32 num)
{
/* We assume the Guest has the same number of GDT entries as the
* Host, otherwise we'd have to dynamically allocate the Guest GDT. */
if (num > ARRAY_SIZE(lg->gdt))
kill_guest(lg, "too many gdt entries %i", num);
/* We read the whole thing in, then fix it up. */
lgread(lg, lg->gdt, table, num * sizeof(lg->gdt[0]));
fixup_gdt_table(lg, 0, ARRAY_SIZE(lg->gdt));
/* Mark that the GDT changed so the core knows it has to copy it again,
* even if the Guest is run on the same CPU. */
lg->changed |= CHANGED_GDT;
}
@ -134,3 +213,13 @@ void guest_load_tls(struct lguest *lg, unsigned long gtls)
fixup_gdt_table(lg, GDT_ENTRY_TLS_MIN, GDT_ENTRY_TLS_MAX+1);
lg->changed |= CHANGED_GDT_TLS;
}
/*
* With this, we have finished the Host.
*
* Five of the seven parts of our task are complete. You have made it through
* the Bit of Despair (I think that's somewhere in the page table code,
* myself).
*
* Next, we examine "make Switcher". It's short, but intense.
*/