Re: [PATCH] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)

["H. Peter Anvin" <hpa@zytor.com>]

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