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

[Denys Vlasenko]

On Mon, Mar 9, 2015 at 5:08 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Mon, Mar 9, 2015 at 7:05 AM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
>>     New:
>>      1e6:   0f ba 64 24 38 11       btl    $0x11,0x38(%esp)
>
> btl? Really?
>
> Why isn't that just
>
>     testb $2,0x3a(%esp)
>
> which is both smaller and quite a bit faster on older machines.

Ok. (I thought people won't be happy about this.)

> 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

These insns must run serially. 4 cycles.

        test
        branch
        test
        branch

can execute both test/branch'es out-of-order in parallel.
2 cycles.
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