From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gleb Natapov Subject: Re: [PATCH] KVM: VMX: Translate interrupt shadow when waiting on NMI window Date: Wed, 21 Apr 2010 17:44:01 +0300 Message-ID: <20100421144401.GF14124@redhat.com> References: <20100216100635.GE2995@redhat.com> <4B7A7015.6000802@siemens.com> <20100216101705.GG2995@redhat.com> <4B7A72FB.8020709@siemens.com> <20100216103235.GH2995@redhat.com> <4B7A755B.80009@siemens.com> <20100216103816.GI2995@redhat.com> <4BCF08DF.8060709@siemens.com> <20100421143007.GE14124@redhat.com> <4BCF0EA2.3010100@siemens.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Avi Kivity , Marcelo Tosatti , kvm To: Jan Kiszka Return-path: Received: from mx1.redhat.com ([209.132.183.28]:28016 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755400Ab0DUOoF (ORCPT ); Wed, 21 Apr 2010 10:44:05 -0400 Content-Disposition: inline In-Reply-To: <4BCF0EA2.3010100@siemens.com> Sender: kvm-owner@vger.kernel.org List-ID: On Wed, Apr 21, 2010 at 04:41:38PM +0200, Jan Kiszka wrote: > Gleb Natapov wrote: > > On Wed, Apr 21, 2010 at 04:17:03PM +0200, Jan Kiszka wrote: > >> Gleb Natapov wrote: > >>> On Tue, Feb 16, 2010 at 11:37:15AM +0100, Jan Kiszka wrote: > >>>> Gleb Natapov wrote: > >>>>> On Tue, Feb 16, 2010 at 11:27:07AM +0100, Jan Kiszka wrote: > >>>>>> Gleb Natapov wrote: > >>>>>>> On Tue, Feb 16, 2010 at 11:14:45AM +0100, Jan Kiszka wrote: > >>>>>>>> Gleb Natapov wrote: > >>>>>>>>> On Tue, Feb 16, 2010 at 11:04:10AM +0100, Jan Kiszka wrote: > >>>>>>>>>> Gleb Natapov wrote: > >>>>>>>>>>> On Tue, Feb 16, 2010 at 10:16:12AM +0100, Jan Kiszka wrote: > >>>>>>>>>>>> Found while browsing Xen code: While we assume that the STI interrupt > >>>>>>>>>>>> shadow also inplies virtual NMI blocking, some processors may have a > >>>>>>>>>>>> different opinion (SDM 3: 22.3). To avoid misunderstandings that would > >>>>>>>>>>>> cause endless VM entry attempts, translate STI into MOV SS blocking when > >>>>>>>>>>>> requesting the NMI window. > >>>>>>>>>>>> > >>>>>>>>>>> Why not just remove "block by STI" check in vmx_nmi_allowed()? IIRC this > >>>>>>>>>>> is documented that on some CPUs STI does not block NMI. > >>>>>>>>>>> > >>>>>>>>>> Probably because we will stumble and fall on those CPUs that do care. > >>>>>>>>>> > >>>>>>>>> But this defines behaviour of cpu _we_ emulate. So on _our_ cpu NMI will > >>>>>>>>> not be blocked by STI. > >>>>>>>> The host CPU decides if it accepts an NMI injections while > >>>>>>> Are you sure? I haven't found such check during VMENTRY. > >>>>>> I also only find the explicitly stated exclusion of MOV SS blocking vs. > >>>>>> NMI injection. If we can rely on this, removing STI blocking from > >>>>>> vmx_nmi_allowed should suffice. Or, better, can we get an official > >>>>>> confirmation from Intel? > >>>>>> > >>>>> SDM 2b says about STI instruction: > >>>>> The IF flag and the STI and CLI instructions do not prohibit the > >>>>> generation of exceptions and NMI interrupts. NMI interrupts (and SMIs) > >>>>> may be blocked for one macroinstruction following an STI. > >>>> Yes, it's likely that this is the architectural reason for the delayed > >>>> NMI window signaling after STI. Still, we are looking for the > >>>> entry-check logic. > >>>> > >>> Will ask Intel. > >>> > >> Just remembered that there was some open topic... Did your ask? Any answer? > >> > > I did and got answer last week :) The answer is that NMI is blocked only > > if GUEST_INTR_STATE_NMI flag is set. MOV SS and STI shouldn't block NMI, > > so vmx_nmi_allowed() should check only GUEST_INTR_STATE_NMI flag. > > Cool, that's now increasing my level of confusion again: :( > > Thought we only wanted to confirm that it's still safe to inject NMIs > when blocked-by-STI is set. Now we hear that it's also safe when MOV SS > is active? That would directly contradict the SDM (at least the version > I have at hand: June 2009). Or did I misunderstand the answer? > No you don't. I was told that software should be prepared to handle NMI after MOV SS. What part of SDM does this contradict? I found nothing in latest SDM. -- Gleb.