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

[Andy Lutomirski]

On Tue, Mar 10, 2015 at 12:57 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Denys Vlasenko <dvlasenk@xxxxxxxxxx> 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.

Before you benchmark, I think you should reorder it: check CS, then
OLDSS, then EFLAGS.  The case where CS & 3 == 0, OLDSS & 4 == 4, and
EFLAGS & VM == VM should be *extremely* rare.

I'm going to hold off on resending my sp0/sp1/ss cleanups until we
resolve this -- if it turns out that your code is the same or faster
and therefore gets merged, then I think that all the -8 crap can just
be deleted instead of being fixed.

--Andy

>
> Thanks,
>
>         Ingo



-- 
Andy Lutomirski
AMA Capital Management, LLC
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