linux-kernel - Re: [PATCH v6 03/20] KVM: x86/pmu: Don't enumerate arch events KVM doesn't support

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Message-ID: <2c804098-af2b-4f1d-a39f-eb42f58635d7@linux.intel.com>
Date:   Tue, 7 Nov 2023 15:14:16 +0800
From:   "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To:     Jim Mattson <jmattson@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Kan Liang <kan.liang@...ux.intel.com>,
        Jinrong Liang <cloudliang@...cent.com>,
        Like Xu <likexu@...cent.com>,
        Aaron Lewis <aaronlewis@...gle.com>
Subject: Re: [PATCH v6 03/20] KVM: x86/pmu: Don't enumerate arch events KVM
 doesn't support


On 11/4/2023 8:41 PM, Jim Mattson wrote:
> On Fri, Nov 3, 2023 at 5:02 PM Sean Christopherson <seanjc@...gle.com> wrote:
>> Don't advertise support to userspace for architectural events that KVM
>> doesn't support, i.e. for "real" events that aren't listed in
>> intel_pmu_architectural_events.  On current hardware, this effectively
>> means "don't advertise support for Top Down Slots".
> NR_REAL_INTEL_ARCH_EVENTS is only used in intel_hw_event_available().
> As discussed (https://lore.kernel.org/kvm/ZUU12-TUR_1cj47u@google.com/),
> intel_hw_event_available() should go away.
>
> Aside from mapping fixed counters to event selector and unit mask
> (fixed_pmc_events[]), KVM has no reason to know when a new
> architectural event is defined.


Since intel_hw_event_available() would be removed, it looks the enum 
intel_pmu_architectural_events and intel_arch_events[] array become 
useless. We can directly simply modify current fixed_pmc_events[] array 
and use it to store fixed counter events code and umask.


>
> The variable that this change "fixes" is only used to feed
> CPUID.0AH:EBX in KVM_GET_SUPPORTED_CPUID, and kvm_pmu_cap.events_mask
> is already constructed from what host perf advertises support for.
>
>> Mask off the associated "unavailable" bits, as said bits for undefined
>> events are reserved to zero.  Arguably the events _are_ defined, but from
>> a KVM perspective they might as well not exist, and there's absolutely no
>> reason to leave useless unavailable bits set.
>>
>> Fixes: a6c06ed1a60a ("KVM: Expose the architectural performance monitoring CPUID leaf")
>> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
>> ---
>>   arch/x86/kvm/vmx/pmu_intel.c | 9 +++++++++
>>   1 file changed, 9 insertions(+)
>>
>> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
>> index 3316fdea212a..8d545f84dc4a 100644
>> --- a/arch/x86/kvm/vmx/pmu_intel.c
>> +++ b/arch/x86/kvm/vmx/pmu_intel.c
>> @@ -73,6 +73,15 @@ static void intel_init_pmu_capability(void)
>>          int i;
>>
>>          /*
>> +        * Do not enumerate support for architectural events that KVM doesn't
>> +        * support.  Clear unsupported events "unavailable" bit as well, as
>> +        * architecturally such bits are reserved to zero.
>> +        */
>> +       kvm_pmu_cap.events_mask_len = min(kvm_pmu_cap.events_mask_len,
>> +                                         NR_REAL_INTEL_ARCH_EVENTS);
>> +       kvm_pmu_cap.events_mask &= GENMASK(kvm_pmu_cap.events_mask_len - 1, 0);
>> +
>> +        /*
>>           * Perf may (sadly) back a guest fixed counter with a general purpose
>>           * counter, and so KVM must hide fixed counters whose associated
>>           * architectural event are unsupported.  On real hardware, this should
>> --
>> 2.42.0.869.gea05f2083d-goog
>>