Re: [PATCH v2 27/36] x86/entry/32: Re-implement SYSENTER using the new C path

[Andy Lutomirski]

On Wed, Oct 7, 2015 at 11:08 AM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
> On 10/06/2015 02:48 AM, Andy Lutomirski wrote:
>> +
>> +/* Opportunistic SYSEXIT */
>> +     TRACE_IRQS_ON                   /* User mode traces as IRQs on. */
>> +     movl    PT_EIP(%esp), %edx      /* pt_regs->ip */
>> +     movl    PT_OLDESP(%esp), %ecx   /* pt_regs->sp */
>> +     popl    %ebx                    /* pt_regs->bx */
>> +     addl    $2*4, %esp              /* skip pt_regs->cx and pt_regs->dx */
>
> Here stack engine and ALUs operating on ESP conflict,
> potentially adding a stall both before and after ADD.
>
> It might be faster to just pop twice into an unused register, say,
>         popl    %eax
>         popl    %eax
>
>> +     popl    %esi                    /* pt_regs->si */
>> +     popl    %edi                    /* pt_regs->di */
>> +     popl    %ebp                    /* pt_regs->bp */
>> +     popl    %eax                    /* pt_regs->ax */
>>  1:   mov     PT_FS(%esp), %fs
>>       PTGS_TO_GS
>

I'll benchmark it and possibly add it at the end of the series.

--Andy
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