From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [RFC] KVM: Fix simultaneous NMIs Date: Mon, 19 Sep 2011 18:09:39 +0300 Message-ID: <4E775B33.3030906@redhat.com> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Jan Kiszka , "kvm@vger.kernel.org" To: Marcelo Tosatti Return-path: Received: from mx1.redhat.com ([209.132.183.28]:41970 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752218Ab1ISPJq (ORCPT ); Mon, 19 Sep 2011 11:09:46 -0400 In-Reply-To: <20110919145403.GB6356@amt.cnet> Sender: kvm-owner@vger.kernel.org List-ID: 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. 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. -- error compiling committee.c: too many arguments to function