From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [PATCH 2/2] KVM: SVM: check for progress after IRET interception Date: Thu, 03 Feb 2011 17:30:21 +0200 Message-ID: <4D4ACA0D.4090707@redhat.com> References: <1296745369-12066-1-git-send-email-avi@redhat.com> <1296745369-12066-3-git-send-email-avi@redhat.com> <4D4AC4B5.7060009@redhat.com> <4D4AC7E3.7070408@siemens.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Marcelo Tosatti , "kvm@vger.kernel.org" , Joerg Roedel To: Jan Kiszka Return-path: Received: from mx1.redhat.com ([209.132.183.28]:41916 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751880Ab1BCPbZ (ORCPT ); Thu, 3 Feb 2011 10:31:25 -0500 In-Reply-To: <4D4AC7E3.7070408@siemens.com> Sender: kvm-owner@vger.kernel.org List-ID: On 02/03/2011 05:21 PM, Jan Kiszka wrote: > > > > So what would be a better fix? We could unconditionally single step on > > iret_interception() which would fix the problem at the cost of making > > NMIs less efficient (three exits instead of two). We could emulate the > > IRET (doubling kvm's code and likely slower, and certainly buggier, than > > the first option). Alternatively, can anyone think of a reliable way to > > make sure forward progress has been made? > > Joerg and I discussed this a few times, I think last on the KVM forum. > It's really tricky and we found no option without limitations. > Single-stepping, e.g., already pollutes the guest state (if an exception > is taken without prior vmexit). We could enable all intercepts when single stepping. if we get a single-step #DB, we clear nmi blocking and are happy. If we get another exception, or a non-single-step #DB, we re-enable the IRET intercept, clear single-step, re-inject the exception, and let the guest continue. > I don't recall all alternatives, but a vmexit-saving one was (IIRC) to > fall back to an interrupt window without IRET interception, likely > augmented with some break-out timer like we do for oldish, vnmi-lacking > Intels. What's an interrupt window without IRET interception? -- error compiling committee.c: too many arguments to function