On 03/09/2015 09:08 AM, Linus Torvalds wrote:
>
> Sure, the btl is easier to explain in the source code, but instead of this:
>
>> + btl $X86_EFLAGS_VM_BIT,PT_EFLAGS(%esp)
>
> you'd have to add a comment, like
>
> testb $2, PT_EFLAGS+2(%esp) # X86_EFLAGS_VM_BIT
>
> or something.
>
Maybe:
testb $(X86_EFLAGS_VM-16), PT_EFLAGS+2(%esp)
> Or just at least *partially* do what we used to do, and make it all be
>
> movb PT_EFLAGS+2(%esp),%al
> andb $2,%al
> orb PT_CS(%esp),%al
> testb $3,%al
> je restore_nocheck
> testb $SEGMENT_TI_MASK,PT_OLDSS(%esp)
> jne ldt_ss
>
> which still avoids looking at SS unless needed, and is smaller and
> faster than the btl, afaik.
The question is if avoiding looking at a field on the stack matters at all.
-hpa
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