From mboxrd@z Thu Jan 1 00:00:00 1970 From: Pekka Enberg Subject: Re: [PATCH 1/2] kvm tools: Respect ISR status in virtio header Date: Sat, 07 May 2011 17:50:34 +0300 Message-ID: <1304779834.10621.14.camel@jaguar> References: <1304735660-10844-1-git-send-email-asias.hejun@gmail.com> <20110507093027.GD27657@elte.hu> <4DC545BA.3030501@us.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit Cc: Ingo Molnar , Asias He , "Michael S. Tsirkin" , Rusty Russell , Mark McLoughlin , Cyrill Gorcunov , Sasha Levin , Prasad Joshi , kvm@vger.kernel.org To: Anthony Liguori Return-path: Received: from filtteri1.pp.htv.fi ([213.243.153.184]:44312 "EHLO filtteri1.pp.htv.fi" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755289Ab1EGOui (ORCPT ); Sat, 7 May 2011 10:50:38 -0400 In-Reply-To: <4DC545BA.3030501@us.ibm.com> Sender: kvm-owner@vger.kernel.org List-ID: On Sat, 2011-05-07 at 08:14 -0500, Anthony Liguori wrote: > It seems like this patch is simply avoiding raising the interrupt line > if the ISR has not been acknowledged yet. I don't think there's a > functional issue here but I'm surprised that it's a win. There should > be a very short window when the interrupt is lowered in the APIC but > still not acknowledged in the ISR. > > You should just be saving a pretty cheap system call. I wonder if the > system call is taking longer than it should.. The patch seems to fix a bug where the guest kernel breaks down under interrupt storm and stops doing VIRTIO_PCI_QUEUE_NOTIFY. We assumed it was something with our code but your comments make me wonder if there's a real problem in KVM_IRQ_LINE. Pekka