Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756756AbbDUWkX (ORCPT ); Tue, 21 Apr 2015 18:40:23 -0400 Received: from mail-la0-f47.google.com ([209.85.215.47]:35754 "EHLO mail-la0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753466AbbDUWkV (ORCPT ); Tue, 21 Apr 2015 18:40:21 -0400 MIME-Version: 1.0 In-Reply-To: References: <1429549782-12962-1-git-send-email-andrew.cooper3@citrix.com> <55359B57.3070008@kernel.org> <20150421124558.GA3483@gmail.com> From: Andy Lutomirski Date: Tue, 21 Apr 2015 15:39:59 -0700 Message-ID: Subject: Re: [RFC PATCH] x86/asm/irq: Don't use POPF but STI To: Linus Torvalds Cc: Ingo Molnar , Andy Lutomirski , Andrew Cooper , Xen-devel , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , "the arch/x86 maintainers" , Linux Kernel Mailing List , Konrad Rzeszutek Wilk , Boris Ostrovsky , David Vrabel , Rusty Russell , lguest@lists.ozlabs.org, Denys Vlasenko , Borislav Petkov Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1364 Lines: 39 On Tue, Apr 21, 2015 at 9:12 AM, Linus Torvalds wrote: > On Tue, Apr 21, 2015 at 5:45 AM, Ingo Molnar 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/