linux-kernel - Re: [RFC PATCH] x86/asm/irq: Don't use POPF but STI

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Message-ID: <CALCETrVzY2vdTcMMbA9w+0AEG0jbRfRhNdO1ta9jwK6_DR=99Q@mail.gmail.com>
Date:	Tue, 21 Apr 2015 15:39:59 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...nel.org>, Andy Lutomirski <luto@...nel.org>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Xen-devel <xen-devel@...ts.xen.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Rusty Russell <rusty@...tcorp.com.au>, lguest@...ts.ozlabs.org,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [RFC PATCH] x86/asm/irq: Don't use POPF but STI

On Tue, Apr 21, 2015 at 9:12 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Tue, Apr 21, 2015 at 5:45 AM, Ingo Molnar <mingo@...nel.org> wrote:
>>
>> Totally untested and not signed off yet: because we'd first have to
>> make sure (via irq flags debugging) that it's not used in reverse, to
>> re-disable interrupts:
>
> Not only might that happen in some place, I *really* doubt that a
> conditional 'sti' is actually any faster. The only way it's going to
> be measurably faster is if you run some microbenchmark so that the
> code is hot and the branch predicts well.
>
> "popf" is fast for the "no changes to IF" case, and is a smaller
> instruction anyway. I'd really hate to make this any more complex
> unless somebody has some real numbers for performance improvement
> (that is *not* just some cycle timing from a bogus test-case, but real
> measurements on a real load).
>
> And even *with* real measurements, I'd worry about the "use popf to
> clear IF" case.

Fair enough.  Maybe I'll benchmark this some day.

--Andy

>
>            Linus



-- 
Andy Lutomirski
AMA Capital Management, LLC
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