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 17:30:07 +0300 Message-ID: <20100421143007.GE14124@redhat.com> References: <20100216100057.GC2995@redhat.com> <4B7A6D9A.5030900@siemens.com> <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> 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]:27136 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754874Ab0DUOaK (ORCPT ); Wed, 21 Apr 2010 10:30:10 -0400 Content-Disposition: inline In-Reply-To: <4BCF08DF.8060709@siemens.com> Sender: kvm-owner@vger.kernel.org List-ID: 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. -- Gleb.