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

["Mi, Dapeng" <dapeng1.mi@linux.intel.com>]

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.
>>
>>