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authorH. Peter Anvin <hpa@linux.intel.com>2012-09-24 16:05:48 -0700
committerH. Peter Anvin <hpa@linux.intel.com>2012-09-26 15:06:22 -0700
commit5a5a51db78ef24aa61a4cb2ae36f07f6fa37356d (patch)
tree22b37361c0d876beeea542f769fdb76444878b36 /arch/x86
parente139e95590dfebab81841bf7a3ac46500f51a47c (diff)
downloadlinux-5a5a51db78ef24aa61a4cb2ae36f07f6fa37356d.tar.bz2
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')
-rw-r--r--arch/x86/kernel/head_32.S31
1 files changed, 16 insertions, 15 deletions
diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index d42ab17b7397..957a47aec64e 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