From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcelo Tosatti Subject: Re: [RFC] KVM: Fix simultaneous NMIs Date: Mon, 19 Sep 2011 12:22:11 -0300 Message-ID: <20110919152211.GA7411@amt.cnet> References: <1316097911-16424-1-git-send-email-avi@redhat.com> <4E722140.4070702@siemens.com> <4E722FA8.2030006@redhat.com> <4E7234F0.2080609@siemens.com> <4E723A8A.7050405@redhat.com> <20110919135421.GA5468@amt.cnet> <4E775203.2020504@redhat.com> <20110919145403.GB6356@amt.cnet> <4E775B33.3030906@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jan Kiszka , "kvm@vger.kernel.org" To: Avi Kivity Return-path: Received: from mx1.redhat.com ([209.132.183.28]:39782 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751619Ab1ISPWg (ORCPT ); Mon, 19 Sep 2011 11:22:36 -0400 Content-Disposition: inline In-Reply-To: <4E775B33.3030906@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Mon, Sep 19, 2011 at 06:09:39PM +0300, Avi Kivity wrote: > On 09/19/2011 05:54 PM, Marcelo Tosatti wrote: > >On Mon, Sep 19, 2011 at 05:30:27PM +0300, Avi Kivity wrote: > >> On 09/19/2011 04:54 PM, Marcelo Tosatti wrote: > >> >> >> > >> >> >> Yes, due to NMI-blocked-by-STI. A really touchy area. > >> >> >And we don't need the window exit notification then? I don't understand > >> >> >what nmi_in_progress is supposed to do here. > >> >> > >> >> We need the window notification in both cases. If we're recovering > >> >> from STI, then we don't need to collapse NMIs. If we're completing > >> >> an NMI handler, then we do need to collapse NMIs (since the queue > >> >> length is two, and we just completed one). > >> > > >> >I don't understand what is the point with nmi_in_progress, and the above > >> >hunk, either. Can't inject_nmi do: > >> > > >> >if (nmi_injected + atomic_read(nmi_pending)< 2) > >> > atomic_inc(nmi_pending) > >> > > >> >Instead of collapsing somewhere else? > >> > >> We could. It's not atomic though - two threads executing in > >> parallel could raise the value to three. Could do a cmpxchg loop > >> does an increment bounded to two. I guess this is a lot clearer, > >> thanks. > >> > >> >You'd also have to change > >> >nmi_injected handling in arch code so its value is not "hidden", in > >> >complete_interrupts(). > >> > >> Or maybe make raising nmi_injected not decrement nmi_pending. So: > >> > >> nmi_pending: total number of interrupts in queue > >> nmi_injected: of these, how many are currently being injected > >> > >> yes? > > > >Yes, at the expense of decrementing on subarch code (which is fine, > >apparently). > > > > Hm, we have no place to decrement. Decrement when setting nmi_injected = false, increment when setting nmi_injected = true, in vmx/svm.c. > We need to do that when IRET > executes, but we don't want to request an NMI window exit in the > common case of nmi_pending = 1. Do not enable nmi window if nmi_injected = true?