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Date: Wed, 18 Mar 2015 22:12:50 +0100
From: Denys Vlasenko <dvlasenk@redhat.com>
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To: Andy Lutomirski <luto@amacapital.net>
CC: Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@kernel.org>,
        Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>, X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/3] x86: entry_64.S: use PUSH insns to build pt_regs
 on stack
References: <1426708056-2785-1-git-send-email-dvlasenk@redhat.com> <1426708056-2785-2-git-send-email-dvlasenk@redhat.com> <CALCETrVhsr1-OcUiWeuU3ATfbYGpYOc=3P96vAwtk=_G2AxsFg@mail.gmail.com>
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On 03/18/2015 10:01 PM, Andy Lutomirski wrote:
> On Wed, Mar 18, 2015 at 12:47 PM, Denys Vlasenko <dvlasenk@redhat.com> 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.