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

[Mi, Dapeng]

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.

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.