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

[Jim Mattson]

On Wed, Nov 1, 2023 at 7:07 PM Mi, Dapeng <dapeng1.mi@xxxxxxxxxxxxxxx> wrote:
>
>
> 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.

The hardware PMU most assuredly does not return 0 for unsupported events.

For example, if I use host perf to sample event selector 0xa4 unit
mask 1 on a Broadwell host (406f1), I get...

# perf stat -e r01a4 sleep 10

 Performance counter stats for 'sleep 10':

           386,964      r01a4

      10.000907211 seconds time elapsed

Broadwell does not advertise support for architectural event 7 in
CPUID.0AH:EBX, so KVM will refuse to measure this event inside a
guest. That seems broken to me.

>
> >
> > 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.
> >>
> >>