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

[hpa]

On March 18, 2019 11:10:22 AM PDT, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>On Mon, Mar 18, 2019 at 10:51 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>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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