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

[Liang, Kan]

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
> 
> 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.
> 
>