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

diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index d42ab17b739729c4b27d3f3fe02fdea2633f2f5a..957a47aec64e12d1833af368c337e4b4e03f56bd 100644 (file)
--- 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