Re: [PATCH] KVM: VMX: Translate interrupt shadow when waiting on NMI window

[Gleb Natapov <gleb@redhat.com>]

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.