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Subject: Re: [PATCH 01/25] x86: Make SMAP 64-bit only
To:     Denys Vlasenko <dvlasenk@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>
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        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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On March 21, 2019 10:25:05 AM PDT, Denys Vlasenko <dvlasenk@redhat.com> wrote:
>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.

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