Re: [PATCH v2] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)

[Ingo Molnar <mingo@kernel.org>]

* Denys Vlasenko <dvlasenk@redhat.com> wrote:

> Old code was trying to avoid having three branch insns,
> but instead it has a chain of six insns where each insn
> depends on previos one.
> 
> And it was touching PT_OLDSS(%esp) unconditionally, even when it may
> contain bogus data. Elsewhere we have to jump thru hoops
> just to make sure here PT_OLDSS(%esp) is at least in a valid page.
> 
> All this just to have one branch instead of three?
> 
> The new code simply checks each condition.
> All three checks can run in parallel on an out-of-order CPU.
> Most of the time, none of branches will be taken.
> 
> Comparison of object code:
>     Old:
>      1e6:   8b 44 24 38             mov    0x38(%esp),%eax
>      1ea:   8a 64 24 40             mov    0x40(%esp),%ah
>      1ee:   8a 44 24 34             mov    0x34(%esp),%al
>      1f2:   25 03 04 02 00          and    $0x20403,%eax
>      1f7:   3d 03 04 00 00          cmp    $0x403,%eax
>      1fc:   74 0f                   je     20d <ldt_ss>
>     New:
>      1e6:   f6 44 24 3a 02          testb  $0x2,0x3a(%esp)
>      1eb:   75 0e                   jne    1fb <restore_nocheck>
>      1ed:   f6 44 24 34 03          testb  $0x3,0x34(%esp)
>      1f2:   74 07                   je     1fb <restore_nocheck>
>      1f4:   f6 44 24 40 04          testb  $0x4,0x40(%esp)
>      1f9:   75 0f                   jne    20a <ldt_ss>

Please do some benchmarking of this: a tight loop of getpid or getppid 
syscalls ought to be enough to be able to time this accurately.

Thanks,

	Ingo