mirror of
https://github.com/AuxXxilium/linux_dsm_epyc7002.git
synced 2024-12-05 15:56:40 +07:00
e4648b014e
Enable TCMU to handle bidirectional SCSI commands. In such cases, entries in iov[] cover both the Data-In and the Data-Out buffers. The first iov_cnt entries correspond to the Data-Out buffer, while the remaining iov_bidi_cnt entries correspond to the Data-In buffer. Signed-off-by: Ilias Tsitsimpis <iliastsi@arrikto.com> Signed-off-by: Vangelis Koukis <vkoukis@arrikto.com> Reviewed-by: Andy Grover <agrover@redhat.com> Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
373 lines
13 KiB
Plaintext
373 lines
13 KiB
Plaintext
Contents:
|
|
|
|
1) TCM Userspace Design
|
|
a) Background
|
|
b) Benefits
|
|
c) Design constraints
|
|
d) Implementation overview
|
|
i. Mailbox
|
|
ii. Command ring
|
|
iii. Data Area
|
|
e) Device discovery
|
|
f) Device events
|
|
g) Other contingencies
|
|
2) Writing a user pass-through handler
|
|
a) Discovering and configuring TCMU uio devices
|
|
b) Waiting for events on the device(s)
|
|
c) Managing the command ring
|
|
3) A final note
|
|
|
|
|
|
TCM Userspace Design
|
|
--------------------
|
|
|
|
TCM is another name for LIO, an in-kernel iSCSI target (server).
|
|
Existing TCM targets run in the kernel. TCMU (TCM in Userspace)
|
|
allows userspace programs to be written which act as iSCSI targets.
|
|
This document describes the design.
|
|
|
|
The existing kernel provides modules for different SCSI transport
|
|
protocols. TCM also modularizes the data storage. There are existing
|
|
modules for file, block device, RAM or using another SCSI device as
|
|
storage. These are called "backstores" or "storage engines". These
|
|
built-in modules are implemented entirely as kernel code.
|
|
|
|
Background:
|
|
|
|
In addition to modularizing the transport protocol used for carrying
|
|
SCSI commands ("fabrics"), the Linux kernel target, LIO, also modularizes
|
|
the actual data storage as well. These are referred to as "backstores"
|
|
or "storage engines". The target comes with backstores that allow a
|
|
file, a block device, RAM, or another SCSI device to be used for the
|
|
local storage needed for the exported SCSI LUN. Like the rest of LIO,
|
|
these are implemented entirely as kernel code.
|
|
|
|
These backstores cover the most common use cases, but not all. One new
|
|
use case that other non-kernel target solutions, such as tgt, are able
|
|
to support is using Gluster's GLFS or Ceph's RBD as a backstore. The
|
|
target then serves as a translator, allowing initiators to store data
|
|
in these non-traditional networked storage systems, while still only
|
|
using standard protocols themselves.
|
|
|
|
If the target is a userspace process, supporting these is easy. tgt,
|
|
for example, needs only a small adapter module for each, because the
|
|
modules just use the available userspace libraries for RBD and GLFS.
|
|
|
|
Adding support for these backstores in LIO is considerably more
|
|
difficult, because LIO is entirely kernel code. Instead of undertaking
|
|
the significant work to port the GLFS or RBD APIs and protocols to the
|
|
kernel, another approach is to create a userspace pass-through
|
|
backstore for LIO, "TCMU".
|
|
|
|
|
|
Benefits:
|
|
|
|
In addition to allowing relatively easy support for RBD and GLFS, TCMU
|
|
will also allow easier development of new backstores. TCMU combines
|
|
with the LIO loopback fabric to become something similar to FUSE
|
|
(Filesystem in Userspace), but at the SCSI layer instead of the
|
|
filesystem layer. A SUSE, if you will.
|
|
|
|
The disadvantage is there are more distinct components to configure, and
|
|
potentially to malfunction. This is unavoidable, but hopefully not
|
|
fatal if we're careful to keep things as simple as possible.
|
|
|
|
Design constraints:
|
|
|
|
- Good performance: high throughput, low latency
|
|
- Cleanly handle if userspace:
|
|
1) never attaches
|
|
2) hangs
|
|
3) dies
|
|
4) misbehaves
|
|
- Allow future flexibility in user & kernel implementations
|
|
- Be reasonably memory-efficient
|
|
- Simple to configure & run
|
|
- Simple to write a userspace backend
|
|
|
|
|
|
Implementation overview:
|
|
|
|
The core of the TCMU interface is a memory region that is shared
|
|
between kernel and userspace. Within this region is: a control area
|
|
(mailbox); a lockless producer/consumer circular buffer for commands
|
|
to be passed up, and status returned; and an in/out data buffer area.
|
|
|
|
TCMU uses the pre-existing UIO subsystem. UIO allows device driver
|
|
development in userspace, and this is conceptually very close to the
|
|
TCMU use case, except instead of a physical device, TCMU implements a
|
|
memory-mapped layout designed for SCSI commands. Using UIO also
|
|
benefits TCMU by handling device introspection (e.g. a way for
|
|
userspace to determine how large the shared region is) and signaling
|
|
mechanisms in both directions.
|
|
|
|
There are no embedded pointers in the memory region. Everything is
|
|
expressed as an offset from the region's starting address. This allows
|
|
the ring to still work if the user process dies and is restarted with
|
|
the region mapped at a different virtual address.
|
|
|
|
See target_core_user.h for the struct definitions.
|
|
|
|
The Mailbox:
|
|
|
|
The mailbox is always at the start of the shared memory region, and
|
|
contains a version, details about the starting offset and size of the
|
|
command ring, and head and tail pointers to be used by the kernel and
|
|
userspace (respectively) to put commands on the ring, and indicate
|
|
when the commands are completed.
|
|
|
|
version - 1 (userspace should abort if otherwise)
|
|
flags - none yet defined.
|
|
cmdr_off - The offset of the start of the command ring from the start
|
|
of the memory region, to account for the mailbox size.
|
|
cmdr_size - The size of the command ring. This does *not* need to be a
|
|
power of two.
|
|
cmd_head - Modified by the kernel to indicate when a command has been
|
|
placed on the ring.
|
|
cmd_tail - Modified by userspace to indicate when it has completed
|
|
processing of a command.
|
|
|
|
The Command Ring:
|
|
|
|
Commands are placed on the ring by the kernel incrementing
|
|
mailbox.cmd_head by the size of the command, modulo cmdr_size, and
|
|
then signaling userspace via uio_event_notify(). Once the command is
|
|
completed, userspace updates mailbox.cmd_tail in the same way and
|
|
signals the kernel via a 4-byte write(). When cmd_head equals
|
|
cmd_tail, the ring is empty -- no commands are currently waiting to be
|
|
processed by userspace.
|
|
|
|
TCMU commands are 8-byte aligned. They start with a common header
|
|
containing "len_op", a 32-bit value that stores the length, as well as
|
|
the opcode in the lowest unused bits. It also contains cmd_id and
|
|
flags fields for setting by the kernel (kflags) and userspace
|
|
(uflags).
|
|
|
|
Currently only two opcodes are defined, TCMU_OP_CMD and TCMU_OP_PAD.
|
|
|
|
When the opcode is CMD, the entry in the command ring is a struct
|
|
tcmu_cmd_entry. Userspace finds the SCSI CDB (Command Data Block) via
|
|
tcmu_cmd_entry.req.cdb_off. This is an offset from the start of the
|
|
overall shared memory region, not the entry. The data in/out buffers
|
|
are accessible via tht req.iov[] array. iov_cnt contains the number of
|
|
entries in iov[] needed to describe either the Data-In or Data-Out
|
|
buffers. For bidirectional commands, iov_cnt specifies how many iovec
|
|
entries cover the Data-Out area, and iov_bidi_cnt specifies how many
|
|
iovec entries immediately after that in iov[] cover the Data-In
|
|
area. Just like other fields, iov.iov_base is an offset from the start
|
|
of the region.
|
|
|
|
When completing a command, userspace sets rsp.scsi_status, and
|
|
rsp.sense_buffer if necessary. Userspace then increments
|
|
mailbox.cmd_tail by entry.hdr.length (mod cmdr_size) and signals the
|
|
kernel via the UIO method, a 4-byte write to the file descriptor.
|
|
|
|
When the opcode is PAD, userspace only updates cmd_tail as above --
|
|
it's a no-op. (The kernel inserts PAD entries to ensure each CMD entry
|
|
is contiguous within the command ring.)
|
|
|
|
More opcodes may be added in the future. If userspace encounters an
|
|
opcode it does not handle, it must set UNKNOWN_OP bit (bit 0) in
|
|
hdr.uflags, update cmd_tail, and proceed with processing additional
|
|
commands, if any.
|
|
|
|
The Data Area:
|
|
|
|
This is shared-memory space after the command ring. The organization
|
|
of this area is not defined in the TCMU interface, and userspace
|
|
should access only the parts referenced by pending iovs.
|
|
|
|
|
|
Device Discovery:
|
|
|
|
Other devices may be using UIO besides TCMU. Unrelated user processes
|
|
may also be handling different sets of TCMU devices. TCMU userspace
|
|
processes must find their devices by scanning sysfs
|
|
class/uio/uio*/name. For TCMU devices, these names will be of the
|
|
format:
|
|
|
|
tcm-user/<hba_num>/<device_name>/<subtype>/<path>
|
|
|
|
where "tcm-user" is common for all TCMU-backed UIO devices. <hba_num>
|
|
and <device_name> allow userspace to find the device's path in the
|
|
kernel target's configfs tree. Assuming the usual mount point, it is
|
|
found at:
|
|
|
|
/sys/kernel/config/target/core/user_<hba_num>/<device_name>
|
|
|
|
This location contains attributes such as "hw_block_size", that
|
|
userspace needs to know for correct operation.
|
|
|
|
<subtype> will be a userspace-process-unique string to identify the
|
|
TCMU device as expecting to be backed by a certain handler, and <path>
|
|
will be an additional handler-specific string for the user process to
|
|
configure the device, if needed. The name cannot contain ':', due to
|
|
LIO limitations.
|
|
|
|
For all devices so discovered, the user handler opens /dev/uioX and
|
|
calls mmap():
|
|
|
|
mmap(NULL, size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0)
|
|
|
|
where size must be equal to the value read from
|
|
/sys/class/uio/uioX/maps/map0/size.
|
|
|
|
|
|
Device Events:
|
|
|
|
If a new device is added or removed, a notification will be broadcast
|
|
over netlink, using a generic netlink family name of "TCM-USER" and a
|
|
multicast group named "config". This will include the UIO name as
|
|
described in the previous section, as well as the UIO minor
|
|
number. This should allow userspace to identify both the UIO device and
|
|
the LIO device, so that after determining the device is supported
|
|
(based on subtype) it can take the appropriate action.
|
|
|
|
|
|
Other contingencies:
|
|
|
|
Userspace handler process never attaches:
|
|
|
|
- TCMU will post commands, and then abort them after a timeout period
|
|
(30 seconds.)
|
|
|
|
Userspace handler process is killed:
|
|
|
|
- It is still possible to restart and re-connect to TCMU
|
|
devices. Command ring is preserved. However, after the timeout period,
|
|
the kernel will abort pending tasks.
|
|
|
|
Userspace handler process hangs:
|
|
|
|
- The kernel will abort pending tasks after a timeout period.
|
|
|
|
Userspace handler process is malicious:
|
|
|
|
- The process can trivially break the handling of devices it controls,
|
|
but should not be able to access kernel memory outside its shared
|
|
memory areas.
|
|
|
|
|
|
Writing a user pass-through handler (with example code)
|
|
-------------------------------------------------------
|
|
|
|
A user process handing a TCMU device must support the following:
|
|
|
|
a) Discovering and configuring TCMU uio devices
|
|
b) Waiting for events on the device(s)
|
|
c) Managing the command ring: Parsing operations and commands,
|
|
performing work as needed, setting response fields (scsi_status and
|
|
possibly sense_buffer), updating cmd_tail, and notifying the kernel
|
|
that work has been finished
|
|
|
|
First, consider instead writing a plugin for tcmu-runner. tcmu-runner
|
|
implements all of this, and provides a higher-level API for plugin
|
|
authors.
|
|
|
|
TCMU is designed so that multiple unrelated processes can manage TCMU
|
|
devices separately. All handlers should make sure to only open their
|
|
devices, based opon a known subtype string.
|
|
|
|
a) Discovering and configuring TCMU UIO devices:
|
|
|
|
(error checking omitted for brevity)
|
|
|
|
int fd, dev_fd;
|
|
char buf[256];
|
|
unsigned long long map_len;
|
|
void *map;
|
|
|
|
fd = open("/sys/class/uio/uio0/name", O_RDONLY);
|
|
ret = read(fd, buf, sizeof(buf));
|
|
close(fd);
|
|
buf[ret-1] = '\0'; /* null-terminate and chop off the \n */
|
|
|
|
/* we only want uio devices whose name is a format we expect */
|
|
if (strncmp(buf, "tcm-user", 8))
|
|
exit(-1);
|
|
|
|
/* Further checking for subtype also needed here */
|
|
|
|
fd = open(/sys/class/uio/%s/maps/map0/size, O_RDONLY);
|
|
ret = read(fd, buf, sizeof(buf));
|
|
close(fd);
|
|
str_buf[ret-1] = '\0'; /* null-terminate and chop off the \n */
|
|
|
|
map_len = strtoull(buf, NULL, 0);
|
|
|
|
dev_fd = open("/dev/uio0", O_RDWR);
|
|
map = mmap(NULL, map_len, PROT_READ|PROT_WRITE, MAP_SHARED, dev_fd, 0);
|
|
|
|
|
|
b) Waiting for events on the device(s)
|
|
|
|
while (1) {
|
|
char buf[4];
|
|
|
|
int ret = read(dev_fd, buf, 4); /* will block */
|
|
|
|
handle_device_events(dev_fd, map);
|
|
}
|
|
|
|
|
|
c) Managing the command ring
|
|
|
|
#include <linux/target_core_user.h>
|
|
|
|
int handle_device_events(int fd, void *map)
|
|
{
|
|
struct tcmu_mailbox *mb = map;
|
|
struct tcmu_cmd_entry *ent = (void *) mb + mb->cmdr_off + mb->cmd_tail;
|
|
int did_some_work = 0;
|
|
|
|
/* Process events from cmd ring until we catch up with cmd_head */
|
|
while (ent != (void *)mb + mb->cmdr_off + mb->cmd_head) {
|
|
|
|
if (tcmu_hdr_get_op(ent->hdr.len_op) == TCMU_OP_CMD) {
|
|
uint8_t *cdb = (void *)mb + ent->req.cdb_off;
|
|
bool success = true;
|
|
|
|
/* Handle command here. */
|
|
printf("SCSI opcode: 0x%x\n", cdb[0]);
|
|
|
|
/* Set response fields */
|
|
if (success)
|
|
ent->rsp.scsi_status = SCSI_NO_SENSE;
|
|
else {
|
|
/* Also fill in rsp->sense_buffer here */
|
|
ent->rsp.scsi_status = SCSI_CHECK_CONDITION;
|
|
}
|
|
}
|
|
else if (tcmu_hdr_get_op(ent->hdr.len_op) != TCMU_OP_PAD) {
|
|
/* Tell the kernel we didn't handle unknown opcodes */
|
|
ent->hdr.uflags |= TCMU_UFLAG_UNKNOWN_OP;
|
|
}
|
|
else {
|
|
/* Do nothing for PAD entries except update cmd_tail */
|
|
}
|
|
|
|
/* update cmd_tail */
|
|
mb->cmd_tail = (mb->cmd_tail + tcmu_hdr_get_len(&ent->hdr)) % mb->cmdr_size;
|
|
ent = (void *) mb + mb->cmdr_off + mb->cmd_tail;
|
|
did_some_work = 1;
|
|
}
|
|
|
|
/* Notify the kernel that work has been finished */
|
|
if (did_some_work) {
|
|
uint32_t buf = 0;
|
|
|
|
write(fd, &buf, 4);
|
|
}
|
|
|
|
return 0;
|
|
}
|
|
|
|
|
|
A final note
|
|
------------
|
|
|
|
Please be careful to return codes as defined by the SCSI
|
|
specifications. These are different than some values defined in the
|
|
scsi/scsi.h include file. For example, CHECK CONDITION's status code
|
|
is 2, not 1.
|