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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>
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        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
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        Steven Rostedt <rostedt@goodmis.org>
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From:   Denys Vlasenko <dvlasenk@redhat.com>
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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.