From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcelo Tosatti Subject: Re: [PATCH 2/2] KVM: SVM: check for progress after IRET interception Date: Tue, 8 Feb 2011 11:49:23 -0200 Message-ID: <20110208134923.GA4412@amt.cnet> References: <1296745369-12066-1-git-send-email-avi@redhat.com> <1296745369-12066-3-git-send-email-avi@redhat.com> <4D4AC4B5.7060009@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: kvm@vger.kernel.org, Jan Kiszka , Joerg Roedel To: Avi Kivity Return-path: Received: from mx1.redhat.com ([209.132.183.28]:57084 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754219Ab1BHOBX (ORCPT ); Tue, 8 Feb 2011 09:01:23 -0500 Content-Disposition: inline In-Reply-To: <4D4AC4B5.7060009@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Thu, Feb 03, 2011 at 05:07:33PM +0200, Avi Kivity wrote: > On 02/03/2011 05:02 PM, Avi Kivity wrote: > >When we enable an NMI window, we ask for an IRET intercept, since > >the IRET re-enables NMIs. However, the IRET intercept happens before > >the instruction executes, while the NMI window architecturally opens > >afterwards. > > > >To compensate for this mismatch, we only open the NMI window in the > >following exit, assuming that the IRET has by then executed; however, > >this assumption is not always correct; we may exit due to a host interrupt > >or page fault, without having executed the instruction. > > > >Fix by checking for forward progress by recording and comparing the IRET's > >rip. This is somewhat of a hack, since an unchaging rip does not mean that > >no forward progress has been made, but is the simplest fix for now. > > Looks good. > 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? Is there other negative impact of the RIP hack than NMI being delayed?