From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=36828 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1Pjun7-000640-5C for qemu-devel@nongnu.org; Mon, 31 Jan 2011 09:31:42 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Pjun2-0002bN-T4 for qemu-devel@nongnu.org; Mon, 31 Jan 2011 09:31:40 -0500 Received: from thoth.sbs.de ([192.35.17.2]:17433) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Pjun2-0002ay-G9 for qemu-devel@nongnu.org; Mon, 31 Jan 2011 09:31:36 -0500 Message-ID: <4D46C7BF.6010809@siemens.com> Date: Mon, 31 Jan 2011 15:31:27 +0100 From: Jan Kiszka MIME-Version: 1.0 References: <4D417F1F.7020302@siemens.com> <4D418230.1010801@siemens.com> <4D4688EB.30408@redhat.com> <4D469C87.3080909@siemens.com> <4D46B7A3.3000106@redhat.com> In-Reply-To: <4D46B7A3.3000106@redhat.com> Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: 7bit Subject: [Qemu-devel] Re: [PATCH v3 14/22] kvm: Fix race between timer signals and vcpu entry under !IOTHREAD List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Avi Kivity Cc: Marcelo Tosatti , "qemu-devel@nongnu.org" , "kvm@vger.kernel.org" , Stefan Hajnoczi On 2011-01-31 14:22, Avi Kivity wrote: > On 01/31/2011 01:27 PM, Jan Kiszka wrote: >> On 2011-01-31 11:03, Avi Kivity wrote: >>> On 01/27/2011 04:33 PM, Jan Kiszka wrote: >>>> Found by Stefan Hajnoczi: There is a race in kvm_cpu_exec between >>>> checking for exit_request on vcpu entry and timer signals arriving >>>> before KVM starts to catch them. Plug it by blocking both timer related >>>> signals also on !CONFIG_IOTHREAD and process those via signalfd. >>>> >>>> As this fix depends on real signalfd support (otherwise the timer >>>> signals only kick the compat helper thread, and the main thread hangs), >>>> we need to detect the invalid constellation and abort configure. >>>> >>>> Signed-off-by: Jan Kiszka >>>> CC: Stefan Hajnoczi >>>> --- >>>> >>>> I don't want to invest that much into !IOTHREAD anymore, so let's see if >>>> the proposed catch&abort is acceptable. >>>> >>> >>> I don't understand the dependency on signalfd. The normal way of doing >>> things, either waiting for the signal in sigtimedwait() or in >>> ioctl(KVM_RUN), works with SIGALRM just fine. >> >> And how would you be kicked out of the select() call if it is waiting >> with a timeout? We only have a single thread here. > > If we use signalfd() (either kernel provided or thread+pipe), we kick > out of select by select()ing it (though I don't see how it works without > an iothread, since an fd can't stop a vcpu unless you enable SIGIO on > it, which is silly for signalfd) > > If you leave it as a naked signal, then it can break out of either > pselect() or vcpu. > > Since the goal is to drop !CONFIG_IOTHREAD, the first path seems better, > I just don't understand the problem with emulated signalfd(). > With the emulated signalfd, there won't be any signal for the VCPU while in KVM_RUN. Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux