Messages in this thread | | | Date | Thu, 2 Nov 2023 10:07:46 +0800 | Subject | Re: [Patch 1/2] KVM: x86/pmu: Add Intel CPUID-hinted TopDown slots event | From | "Mi, Dapeng" <> |
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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.
> > 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. >> >>
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