linux-kernel - Re: [PATCH v2 1/6] KVM: x86/pmu: Setup pmc->eventsel for fixed PMCs

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Message-ID: <7ca8ffcb-eb45-a47c-f91b-6dbd35ea8893@gmail.com>
Date:   Tue, 7 Dec 2021 14:07:55 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Like Xu <likexu@...cent.com>
Subject: Re: [PATCH v2 1/6] KVM: x86/pmu: Setup pmc->eventsel for fixed PMCs

On 7/12/2021 3:50 am, Jim Mattson wrote:
> On Mon, Nov 29, 2021 at 11:42 PM Like Xu <like.xu.linux@...il.com> wrote:
> 
>> From: Like Xu <likexu@...cent.com>
>>
>> The current pmc->eventsel for fixed counter is underutilised. The
>> pmc->eventsel can be setup for all known available fixed counters
>> since we have mapping between fixed pmc index and
>> the intel_arch_events array.
>>
>> Either gp or fixed counter, it will simplify the later checks for
>> consistency between eventsel and perf_hw_id.
>>
>> Signed-off-by: Like Xu <likexu@...cent.com>
>> ---
>>   arch/x86/kvm/vmx/pmu_intel.c | 16 ++++++++++++++++
>>   1 file changed, 16 insertions(+)
>>
>> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
>> index 1b7456b2177b..b7ab5fd03681 100644
>> --- a/arch/x86/kvm/vmx/pmu_intel.c
>> +++ b/arch/x86/kvm/vmx/pmu_intel.c
>> @@ -459,6 +459,21 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu,
>> struct msr_data *msr_info)
>>          return 1;
>>   }
>>
>> +static void setup_fixed_pmc_eventsel(struct kvm_pmu *pmu)
>> +{
>> +       size_t size = ARRAY_SIZE(fixed_pmc_events);
>> +       struct kvm_pmc *pmc;
>> +       u32 event;
>> +       int i;
>> +
>> +       for (i = 0; i < pmu->nr_arch_fixed_counters; i++) {
>> +               pmc = &pmu->fixed_counters[i];
>> +               event = fixed_pmc_events[array_index_nospec(i, size)];
>>
> 
> How do we know that i < size? For example, Ice Lake supports 4
> fixed counters, but fixed_pmc_events only has three entries.

With the help of macro MAX_FIXED_COUNTERS,
the fourth or more fixed counter is currently not supported in KVM.

If the user space sets a super set of CPUID supported by KVM,
any pmu emulation failure is to be expected, right ?

Waiting for more comments from you on this patch set.

> 
> 
>> +               pmc->eventsel = (intel_arch_events[event].unit_mask << 8) |
>> +                       intel_arch_events[event].eventsel;
>> +       }
>> +}
>> +
>>
>>
>>
>