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Subject: Re: [PATCH 01/25] x86: Make SMAP 64-bit only
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>, Peter Anvin <hpa@zytor.com>,
        Julien Thierry <julien.thierry@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Ingo Molnar <mingo@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        James Morse <james.morse@arm.com>, valentin.schneider@arm.com,
        Brian Gerst <brgerst@gmail.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Andrew Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        Steven Rostedt <rostedt@goodmis.org>
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From:   Denys Vlasenko <dvlasenk@redhat.com>
Message-ID: <aac45ef8-0196-b1c4-67ce-26dd0da6d641@redhat.com>
Date:   Thu, 21 Mar 2019 18:25:05 +0100
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On 3/18/19 7:10 PM, Linus Torvalds wrote:
> On Mon, Mar 18, 2019 at 10:51 AM Peter Zijlstra <peterz@infradead.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.