diff options
| author | H. Peter Anvin <hpa@linux.intel.com> | 2012-09-24 16:05:48 -0700 |
|---|---|---|
| committer | H. Peter Anvin <hpa@linux.intel.com> | 2012-09-26 15:06:22 -0700 |
| commit | 5a5a51db78ef24aa61a4cb2ae36f07f6fa37356d (patch) | |
| tree | 22b37361c0d876beeea542f769fdb76444878b36 /arch/x86/kernel | |
| parent | e139e95590dfebab81841bf7a3ac46500f51a47c (diff) | |
x86-32: Start out eflags and cr4 clean
%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
Diffstat (limited to 'arch/x86/kernel')
| -rw-r--r-- | arch/x86/kernel/head_32.S | 31 |
1 files changed, 16 insertions, 15 deletions
diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S index d42ab17b739..957a47aec64 100644 --- a/arch/x86/kernel/head_32.S +++ b/arch/x86/kernel/head_32.S @@ -287,27 +287,28 @@ ENTRY(startup_32_smp) leal -__PAGE_OFFSET(%ecx),%esp default_entry: - /* * New page tables may be in 4Mbyte page mode and may * be using the global pages. * * 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 - * some bits in it to set. This won't work if the BSP - * 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.. + * Specifically, cr4 exists if and only if CPUID exists, + * which in turn exists if and only if EFLAGS.ID exists. */ -#define cr4_bits pa(mmu_cr4_features) - movl cr4_bits,%edx - andl %edx,%edx - jz 6f - movl %cr4,%eax # Turn on paging options (PSE,PAE,..) - orl %edx,%eax + movl $X86_EFLAGS_ID,%ecx + pushl %ecx + popfl + pushfl + popl %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 testb $X86_CR4_PAE, %al # check if PAE is enabled |