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

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Date:   Mon, 18 Mar 2019 11:10:22 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>, Peter Anvin <hpa@...or.com>,
        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 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