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

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Message-ID: <CA+55aFyB+osF98q8ZUoWset3WvTTc-32dhe4ozgANdqi68XD2A@mail.gmail.com>
Date:	Tue, 21 Apr 2015 09:12:09 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	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 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.

           Linus
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