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

[Gleb Natapov <gleb@redhat.com>]

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.

--
			Gleb.