linux-kernel - Re: RFC: Petition Intel/AMD to add POPF

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Date:	Wed, 17 Aug 2016 12:13:27 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Sara Sharon <sara.sharon@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Christian König <christian.koenig@....com>,
	Vinod Koul <vinod.koul@...el.com>,
	Alex Deucher <alexander.deucher@....com>,
	Johannes Berg <johannes.berg@...el.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Andy Lutomirski <luto@...nel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Adrian Hunter <adrian.hunter@...el.com>
Subject: Re: RFC: Petition Intel/AMD to add POPF_IF insn

On Wed, Aug 17, 2016 at 12:01 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Aug 17, 2016 11:41 AM, "Denys Vlasenko" <dvlasenk@...hat.com> wrote:
>>
>> OTOH 5 years will inevitably pass.
>
> Yes. But in five years, maybe we'll have a popf that is faster anyway.
>
> I'd actually prefer that in the end. The problem with popf right now seems
> to be mainly that it's effectively serializing and does stupid things in
> microcode. It doesn't have to be that way. It could actually do much better,
> but it hasn't been a high enough priority for Intel.
>

It wouldn't surprise me if that were easier said than done.  popf
potentially changes AC, and AC affects address translation.  popf also
potentially changes IOPL, and I don't know whether Intel chips track
IOPL in a way that lets them find all the dependent instructions
without serializing.  But maybe their pipeline is fancy enough.

Personally, I still expect that a simple branch-and-sti is the way to
go.  It wouldn't shock me if even a mispredicted branch and STI is
faster than POPF.