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

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Message-ID: <baa64cf4-11de-4581-89b6-3a86448e3a6e@linux.intel.com>
Date:   Wed, 1 Nov 2023 11:31:46 +0800
From:   "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        Zhang Xiong <xiong.y.zhang@...el.com>,
        Mingwei Zhang <mizhang@...gle.com>,
        Like Xu <like.xu.linux@...il.com>,
        Dapeng Mi <dapeng1.mi@...el.com>, Like Xu <likexu@...cent.com>,
        Kan Liang <kan.liang@...ux.intel.com>
Subject: Re: [Patch 1/2] KVM: x86/pmu: Add Intel CPUID-hinted TopDown slots
 event


On 11/1/2023 11:04 AM, Jim Mattson wrote:
> On Tue, Oct 31, 2023 at 6:59 PM Mi, Dapeng <dapeng1.mi@...ux.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@...ux.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.

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.