From owner-svn-src-head@FreeBSD.ORG Mon May 25 17:16:23 2015 Return-Path: Delivered-To: svn-src-head@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [8.8.178.115]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTPS id 3BBDAB8D; Mon, 25 May 2015 17:16:23 +0000 (UTC) (envelope-from jhb@freebsd.org) Received: from bigwig.baldwin.cx (bigwig.baldwin.cx [IPv6:2001:470:1f11:75::1]) (using TLSv1 with cipher DHE-RSA-CAMELLIA256-SHA (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 15034BB8; Mon, 25 May 2015 17:16:23 +0000 (UTC) (envelope-from jhb@freebsd.org) Received: from ralph.baldwin.cx (pool-173-54-116-245.nwrknj.fios.verizon.net [173.54.116.245]) by bigwig.baldwin.cx (Postfix) with ESMTPSA id 2E82FB926; Mon, 25 May 2015 13:16:21 -0400 (EDT) From: John Baldwin To: Ian Lepore Cc: Andrew Turner , Andrew Turner , src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r283331 - head/sys/arm/arm Date: Mon, 25 May 2015 12:33:48 -0400 Message-ID: <1756258.Dk05pRRcTN@ralph.baldwin.cx> User-Agent: KMail/4.14.3 (FreeBSD/10.1-STABLE; KDE/4.14.3; amd64; ; ) In-Reply-To: <1432566566.1200.37.camel@freebsd.org> References: <201505232228.t4NMSxs2032365@svn.freebsd.org> <3083392.nvUXfWWOav@ralph.baldwin.cx> <1432566566.1200.37.camel@freebsd.org> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.2.7 (bigwig.baldwin.cx); Mon, 25 May 2015 13:16:21 -0400 (EDT) X-BeenThere: svn-src-head@freebsd.org X-Mailman-Version: 2.1.20 Precedence: list List-Id: SVN commit messages for the src tree for head/-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 25 May 2015 17:16:23 -0000 On Monday, May 25, 2015 09:09:26 AM Ian Lepore wrote: > On Mon, 2015-05-25 at 10:31 -0400, John Baldwin wrote: > > Mmmm, does that mean then that you can (conceivably) lose the race the other > > way where it "sees" ap_ready's update before it calls wfe and never calls > > wfe to "harvest" the event from sev? (In practice I think this is not > > possible during boot as AP's can't get preempted and there is typically > > a "long" time between AP's being signalled to start and start_aps being > > set. However, this would be a concern for use of wfe/sev for other use > > cases such as for the cpu_idle hook perhaps?) > > > > That's the "you must be prepared to handle spurious wakeups" part of the > sev/wfe contract. The point of WFE is only power-saving, so if your > loop spins one time due to an unharvested prior event flag still set, > that's deemed harmless. (Userland is allowed to issue SEV instructions, > which always target all cores, so there's no expectation of 1:1 relation > between sending and waiting.) Ok, good to know. Thanks! -- John Baldwin