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SubjectRe: [PATCH v2] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)
On Tue, Mar 10, 2015 at 12:57 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * 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.

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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