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

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Date:   Thu, 21 Mar 2019 10:12:35 -0700
From:   hpa@...or.com
To:     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>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        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 18, 2019 11:10:22 AM PDT, Linus Torvalds <torvalds@...ux-foundation.org> 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. Which would be the common
>case unless you'd also make the popf do the irq restore part and
>simply make finish_lock_switch() re-enable irq's by doing an
>irqrestore?
>
>I think popf is like 20 cycles or something (and pushf is just single
>cycles). Probably not worth worrying about in the task switch context.
>
>                 Linus

Yes, the problem isn't scheduling per se but the risk of hiding problems.
-- 
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