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

[Sean Christopherson <seanjc@google.com>]

On Fri, Nov 03, 2023, Jim Mattson wrote:
> On Fri, Nov 3, 2023 at 8:13 AM Liang, Kan <kan.liang@linux.intel.com> wrote:
> >
> >
> >
> > On 2023-11-02 1:45 p.m., Jim Mattson wrote:
> > > On Wed, Nov 1, 2023 at 7:07 PM Mi, Dapeng <dapeng1.mi@linux.intel.com> 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@linux.intel.com> wrote:
> > >>>>>> On 11/1/2023 2:22 AM, Jim Mattson wrote:
> > >>>>>>> On Tue, Oct 31, 2023 at 1:58 AM Dapeng Mi
> > >>>>>>> <dapeng1.mi@linux.intel.com> 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...
> >
> > I think we have different understanding about the meaning of the
> > "unsupported". There is no enumeration of the Architectural Topdown
> > Slots, which only means the Topdown Slots/01a4 is not an architectural
> > event on the platform. It doesn't mean that the event encoding is
> > unsupported. It could be used by another event, especially on the
> > previous platform.
> 
> If the same event encoding could be used by a microarchitectural event
> on a prior platform, then it is *definitely* wrong for KVM to refuse
> to monitor the event just because it isn't enumerated as a supported
> architectural event.

+1000!  Thanks Kan, this is exactly the info we need!

I'll add a patch to build on "Always treat Fixed counters as available when
supported"[*] and rip out intel_hw_event_available().

[*] https://lore.kernel.org/all/20231024002633.2540714-4-seanjc@google.com