Commit-ID: 5a5a51db78ef24aa61a4cb2ae36f07f6fa37356d Gitweb: http://git.kernel.org/tip/5a5a51db78ef24aa61a4cb2ae36f07f6fa37356d Author: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx> AuthorDate: Mon, 24 Sep 2012 16:05:48 -0700 Committer: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx> CommitDate: Wed, 26 Sep 2012 15:06:22 -0700 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@xxxxxxxxxxxxxxx> Link: http://lkml.kernel.org/r/1348529239-17943-1-git-send-email-hpa@xxxxxxxxxxxxxxx --- 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 d42ab17..957a47a 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 -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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