From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kiszka Subject: Re: [PATCH] KVM: VMX: Translate interrupt shadow when waiting on NMI window Date: Wed, 21 Apr 2010 16:41:38 +0200 Message-ID: <4BCF0EA2.3010100@siemens.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> <20100421143007.GE14124@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Avi Kivity , Marcelo Tosatti , kvm To: Gleb Natapov Return-path: Received: from thoth.sbs.de ([192.35.17.2]:15125 "EHLO thoth.sbs.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755302Ab0DUOl5 (ORCPT ); Wed, 21 Apr 2010 10:41:57 -0400 In-Reply-To: <20100421143007.GE14124@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: 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? Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux