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x86-32: Start out eflags and cr4 clean

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%cr4 is supposed to reflect a set of features into which the operating
system is opting in.  If the BIOS or bootloader leaks bits here, this
is not desirable.  Consider a bootloader passing in %cr4.pae set to a
legacy paging kernel, for example -- it will not have any immediate
effect, but the kernel would crash when turning paging on.

A similar argument applies to %eflags, and since we have to look for
%eflags.id being settable we can use a sequence which clears %eflags
as a side effect.

Note that we already do this for x86-64.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Link: http://lkml.kernel.org/r/1348529239-17943-1-git-send-email-hpa@linux.intel.com
This commit is contained in:
H. Peter Anvin 2012-09-24 16:05:48 -07:00
parent e139e95590
commit 5a5a51db78
1 changed files with 16 additions and 15 deletions
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31
arch/x86/kernel/head_32.S
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@ -287,27 +287,28 @@ ENTRY(startup_32_smp)
leal -__PAGE_OFFSET(%ecx),%esp leal -__PAGE_OFFSET(%ecx),%esp
default_entry: default_entry:
/* /*
* New page tables may be in 4Mbyte page mode and may * New page tables may be in 4Mbyte page mode and may
* be using the global pages. * be using the global pages.
* *
* NOTE! If we are on a 486 we may have no cr4 at all! * NOTE! If we are on a 486 we may have no cr4 at all!
* So we do not try to touch it unless we really have * Specifically, cr4 exists if and only if CPUID exists,
* some bits in it to set. This won't work if the BSP * which in turn exists if and only if EFLAGS.ID exists.
* implements cr4 but this AP does not -- very unlikely
* but be warned! The same applies to the pse feature
* if not equally supported. --macro
*
* NOTE! We have to correct for the fact that we're
* not yet offset PAGE_OFFSET..
*/ */
#define cr4_bits pa(mmu_cr4_features) movl $X86_EFLAGS_ID,%ecx
movl cr4_bits,%edx pushl %ecx
andl %edx,%edx popfl
jz 6f pushfl
movl %cr4,%eax # Turn on paging options (PSE,PAE,..) popl %eax
orl %edx,%eax pushl $0
popfl
pushfl
popl %edx
xorl %edx,%eax
testl %ecx,%eax
jz 6f # No ID flag = no CPUID = no CR4
movl pa(mmu_cr4_features),%eax
movl %eax,%cr4 movl %eax,%cr4
testb $X86_CR4_PAE, %al # check if PAE is enabled testb $X86_CR4_PAE, %al # check if PAE is enabled