Re: [PATCH 2/3] x86: entry_64.S: use PUSH insns to build pt_regs on stack

From: Andy Lutomirski
Date: Wed Mar 18 2015 - 17:22:34 EST

On Wed, Mar 18, 2015 at 2:12 PM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
> On 03/18/2015 10:01 PM, Andy Lutomirski wrote:
>> On Wed, Mar 18, 2015 at 12:47 PM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
>>> We lose a number of large insns there:
>>> text data bss dec hex filename
>>> 9863 0 0 9863 2687 entry_64_before.o
>>> 9671 0 0 9671 25c7 entry_64.o
>>> What's more important, we convert two "MOVQ $imm,off(%rsp)" to "PUSH $imm"
>>> (the ones which fill pt_regs->cs,ss).
>>> Before this patch, placing them on fast path was slowing it down by two cycles:
>>> this form of MOV is very large, 12 bytes, and this probably reduces decode bandwidth
>>> to one insn per cycle when it meets them.
>>> Therefore they were living in FIXUP_TOP_OF_STACK instead (away from hot path).
>> Does that mean that this has zero performance impact, or is it
>> actually a speedup?
> No, it's not a speedup because those big bad instructions weren't
> on hot path to begin with.
> We want them to be there.
> Inserting them in a form of MOVs into hot path (say, in order
> to eliminate FIXUP_TOP_OF_STACK) *would be* a slowdown.
> But we switch to PUSH method, and then inserting them _as PUSHes_
> seems to be a wash.

Sorry, what I meant was: what was the performance impact of this patch
on fast-path syscalls?

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