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

[Denys Vlasenko]

On 3/18/19 7:10 PM, Linus Torvalds 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.

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.