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Commit 5a5a51db authored by H. Peter Anvin's avatar H. Peter Anvin
Browse files

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: default avatarH. Peter Anvin <hpa@linux.intel.com>
Link: http://lkml.kernel.org/r/1348529239-17943-1-git-send-email-hpa@linux.intel.com
parent e139e955

arch/x86/kernel/head_32.S

+16 −15
Original line number Diff line number Diff line
@@ -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