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SubjectRe: [PATCH 2/5 -v2.1] x86: Detect CPUID support early at boot
On 02/09/2013 03:16 PM, Borislav Petkov wrote:
> From: Borislav Petkov <bp@suse.de>
>
> We detect CPUID function support on each CPU and save it for later use,
> obviating the need to play the toggle EFLAGS.ID game every time. C code
> is looking at ->cpuid_level anyway.
>
> Signed-off-by: Borislav Petkov <bp@suse.de>
> ---
> arch/x86/kernel/head_32.S | 48 +++++++++++++++++++++++------------------------
> 1 file changed, 23 insertions(+), 25 deletions(-)
>
> diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
> index f4d919e2cd2b..534397ba226c 100644
> --- a/arch/x86/kernel/head_32.S
> +++ b/arch/x86/kernel/head_32.S
> @@ -318,30 +318,37 @@ default_entry:
> movl %eax,%cr0
>
> /*
> - * New page tables may be in 4Mbyte page mode and may
> - * be using the global pages.
> + * Initialize EFLAGS. Some BIOSes leave bits like NT set. This would confuse the
> + * debugger if this code is traced. Best to initialize before switching to
> + * protected mode. As a side effect, we clear DF too because GCC expects it so.
> + */
> + pushl $0
> + popfl
> +

I wouldn't really call it a "side effect". Perhaps the right thing here
is to say something like "we want to start out with %eflags
unambiguously clear".

(Note also we have had to CLD earlier because we have already copied the
command line.)

-hpa

--
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel. I don't speak on their behalf.



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