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
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1 changed files with 16 additions and 15 deletions
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@ -287,27 +287,28 @@ ENTRY(startup_32_smp)
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leal -__PAGE_OFFSET(%ecx),%esp
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default_entry:
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/*
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* New page tables may be in 4Mbyte page mode and may
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* be using the global pages.
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*
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* NOTE! If we are on a 486 we may have no cr4 at all!
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* So we do not try to touch it unless we really have
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* some bits in it to set. This won't work if the BSP
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* implements cr4 but this AP does not -- very unlikely
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* but be warned! The same applies to the pse feature
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* if not equally supported. --macro
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*
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* NOTE! We have to correct for the fact that we're
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* not yet offset PAGE_OFFSET..
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* Specifically, cr4 exists if and only if CPUID exists,
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* which in turn exists if and only if EFLAGS.ID exists.
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*/
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#define cr4_bits pa(mmu_cr4_features)
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movl cr4_bits,%edx
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andl %edx,%edx
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jz 6f
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movl %cr4,%eax # Turn on paging options (PSE,PAE,..)
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orl %edx,%eax
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movl $X86_EFLAGS_ID,%ecx
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pushl %ecx
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popfl
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pushfl
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popl %eax
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pushl $0
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popfl
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pushfl
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popl %edx
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xorl %edx,%eax
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testl %ecx,%eax
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jz 6f # No ID flag = no CPUID = no CR4
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movl pa(mmu_cr4_features),%eax
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movl %eax,%cr4
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testb $X86_CR4_PAE, %al # check if PAE is enabled
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