Re: [Patch 1/2] KVM: x86/pmu: Add Intel CPUID-hinted TopDown slots event

[Mi, Dapeng]

On 11/1/2023 9:33 PM, Liang, Kan wrote:
> On 2023-10-31 11:31 p.m., Mi, Dapeng wrote:
> > On 11/1/2023 11:04 AM, Jim Mattson wrote:
> > > On Tue, Oct 31, 2023 at 6:59 PM Mi, Dapeng
> > > <dapeng1.mi@xxxxxxxxxxxxxxx> wrote:
> > > > On 11/1/2023 2:22 AM, Jim Mattson wrote:
> > > > > On Tue, Oct 31, 2023 at 1:58 AM Dapeng Mi
> > > > > <dapeng1.mi@xxxxxxxxxxxxxxx> wrote:
> > > > > > This patch adds support for the architectural topdown slots event
> > > > > > which
> > > > > > is hinted by CPUID.0AH.EBX.
> > > > > Can't a guest already program an event selector to count event select
> > > > > 0xa4, unit mask 1, unless the event is prohibited by
> > > > > KVM_SET_PMU_EVENT_FILTER?
> > > > Actually defining this new slots arch event is to do the sanity check
> > > > for supported arch-events which is enumerated by CPUID.0AH.EBX.
> > > > Currently vPMU would check if the arch event from guest is supported by
> > > > KVM. If not, it would be rejected just like intel_hw_event_available()
> > > > shows.
> > > >
> > > > If we don't add the slots event in the intel_arch_events[] array, guest
> > > > may program the slots event and pass the sanity check of KVM on a
> > > > platform which actually doesn't support slots event and program the
> > > > event on a real GP counter and got an invalid count. This is not
> > > > correct.
> > > On physical hardware, it is possible to program a GP counter with the
> > > event selector and unit mask of the slots event whether or not the
> > > platform supports it. Isn't KVM wrong to disallow something that a
> > > physical CPU allows?
> >
> > Yeah, I agree. But I'm not sure if this is a flaw on PMU driver. If an
> > event is not supported by the hardware,  we can't predict the PMU's
> > behavior and a meaningless count may be returned and this could mislead
> > the user.
> The user can program any events on the GP counter. The perf doesn't
> limit it. For the unsupported event, 0 should be returned. Please keep
> in mind, the event list keeps updating. If the kernel checks for each
> event, it could be a disaster. I don't think it's a flaw.


Thanks Kan, it would be ok as long as 0 is always returned for 
unsupported events. IMO, it's a nice to have feature that KVM does this 
sanity check for supported arch events, it won't break anything.


> Thanks,
> Kan
> > Add Kan to confirm this.
> >
> > Hi Kan,
> >
> > Have you any comments on this? Thanks.
> >
> >
> > > > > AFAICT, this change just enables event filtering based on
> > > > > CPUID.0AH:EBX[bit 7] (though it's not clear to me why two independent
> > > > > mechanisms are necessary for event filtering).
> > > > IMO, these are two different things. this change is just to enable the
> > > > supported arch events check for slot events, the event filtering is
> > > > another thing.
> > > How is clearing CPUID.0AH:EBX[bit 7] any different from putting {event
> > > select 0xa4, unit mask 1} in a deny list with the PMU event filter?
> > I think there is no difference in the conclusion but with two different
> > methods.