linux-kernel - Re: [PATCH 01/25] x86: Make SMAP 64-bit only

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Date:   Thu, 21 Mar 2019 11:18:05 -0700
From:   hpa@...or.com
To:     Denys Vlasenko <dvlasenk@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Julien Thierry <julien.thierry@....com>,
        Will Deacon <will.deacon@....com>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>, valentin.schneider@....com,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 01/25] x86: Make SMAP 64-bit only

On March 21, 2019 10:25:05 AM PDT, Denys Vlasenko <dvlasenk@...hat.com> wrote:
>On 3/18/19 7:10 PM, Linus Torvalds wrote:
>> On Mon, Mar 18, 2019 at 10:51 AM Peter Zijlstra
><peterz@...radead.org> wrote:
>>>
>>> How about I do a patch that schedules EFLAGS for both 32bit and
>64bit,
>>> mark this for backporting to infinity.
>>>
>>> And then at the end, after the objtool-ac bits land, I do a patch
>>> removing the EFLAGS scheduling for x86_64.
>> 
>> Sounds sane to me.
>> 
>> And we can make it AC-conditional if it's actually shown to be
>visible
>> from a performance standpoint.
>> 
>> But iirc pushf/popf isn't really that expensive - in fact I think
>it's
>> pretty cheap when system flags don't change.
>
>I did not see evidence of this. In my testing,
>POPF is always ~20 cycles, even if popped flags are identical to
>current
>state of flags.

I think you will find that if you change system flags it is much slower.
-- 
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