linux-kernel - Re: [PATCH 1/4] KVM: x86/svm/pmu: Limit the maximum number of supported GP counters

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Message-ID: <309cbd6b-4d50-7a11-f7c1-0859b0f812d1@gmail.com>
Date:   Tue, 6 Sep 2022 20:38:18 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sandipan Das <sandipan.das@....com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] KVM: x86/svm/pmu: Limit the maximum number of
 supported GP counters

On 6/9/2022 1:26 am, Jim Mattson wrote:
> On Mon, Sep 5, 2022 at 5:45 AM Like Xu <like.xu.linux@...il.com> wrote:
>>
>> From: Like Xu <likexu@...cent.com>
>>
>> The AMD PerfMonV2 specification allows for a maximum of 16 GP counters,
>> which is clearly not supported with zero code effort in the current KVM.
>>
>> A local macro (named like INTEL_PMC_MAX_GENERIC) is introduced to
>> take back control of this virt capability, which also makes it easier to
>> statically partition all available counters between hosts and guests.
>>
>> Signed-off-by: Like Xu <likexu@...cent.com>
>> ---
>>   arch/x86/kvm/pmu.h     | 2 ++
>>   arch/x86/kvm/svm/pmu.c | 7 ++++---
>>   arch/x86/kvm/x86.c     | 2 ++
>>   3 files changed, 8 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
>> index 847e7112a5d3..e3a3813b6a38 100644
>> --- a/arch/x86/kvm/pmu.h
>> +++ b/arch/x86/kvm/pmu.h
>> @@ -18,6 +18,8 @@
>>   #define VMWARE_BACKDOOR_PMC_REAL_TIME          0x10001
>>   #define VMWARE_BACKDOOR_PMC_APPARENT_TIME      0x10002
>>
>> +#define KVM_AMD_PMC_MAX_GENERIC        AMD64_NUM_COUNTERS_CORE
>> +
>>   struct kvm_event_hw_type_mapping {
>>          u8 eventsel;
>>          u8 unit_mask;
>> diff --git a/arch/x86/kvm/svm/pmu.c b/arch/x86/kvm/svm/pmu.c
>> index 2ec420b85d6a..f99f2c869664 100644
>> --- a/arch/x86/kvm/svm/pmu.c
>> +++ b/arch/x86/kvm/svm/pmu.c
>> @@ -192,9 +192,10 @@ static void amd_pmu_init(struct kvm_vcpu *vcpu)
>>          struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
>>          int i;
>>
>> -       BUILD_BUG_ON(AMD64_NUM_COUNTERS_CORE > INTEL_PMC_MAX_GENERIC);
>> +       BUILD_BUG_ON(AMD64_NUM_COUNTERS_CORE > KVM_AMD_PMC_MAX_GENERIC);
>> +       BUILD_BUG_ON(KVM_AMD_PMC_MAX_GENERIC > INTEL_PMC_MAX_GENERIC);
>>
>> -       for (i = 0; i < AMD64_NUM_COUNTERS_CORE ; i++) {
>> +       for (i = 0; i < KVM_AMD_PMC_MAX_GENERIC ; i++) {
>>                  pmu->gp_counters[i].type = KVM_PMC_GP;
>>                  pmu->gp_counters[i].vcpu = vcpu;
>>                  pmu->gp_counters[i].idx = i;
>> @@ -207,7 +208,7 @@ static void amd_pmu_reset(struct kvm_vcpu *vcpu)
>>          struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
>>          int i;
>>
>> -       for (i = 0; i < AMD64_NUM_COUNTERS_CORE; i++) {
>> +       for (i = 0; i < KVM_AMD_PMC_MAX_GENERIC; i++) {
>>                  struct kvm_pmc *pmc = &pmu->gp_counters[i];
>>
>>                  pmc_stop_counter(pmc);
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 43a6a7efc6ec..b9738efd8425 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -1444,12 +1444,14 @@ static const u32 msrs_to_save_all[] = {
>>          MSR_ARCH_PERFMON_EVENTSEL0 + 16, MSR_ARCH_PERFMON_EVENTSEL0 + 17,
> 
> IIRC, the effective maximum on the Intel side is 18, despite what
> INTEL_PMC_MAX_GENERIC says, due to collisions with other existing MSR

Emm, check https://lore.kernel.org/kvm/20220906081604.24035-1-likexu@tencent.com/

> indices. That's why the Intel list stops here. Should we introduce a
> KVM_INTEL_PMC_MAX_GENERIC as well?

Yes, this suggestion will be applied in the next version.

> 
>>          MSR_IA32_PEBS_ENABLE, MSR_IA32_DS_AREA, MSR_PEBS_DATA_CFG,
>>
>> +       /* This part of MSRs should match KVM_AMD_PMC_MAX_GENERIC. */
> 
> Perhaps the comment above should be moved down two lines, since the
> next two lines deal with the legacy counters.

Applied, thanks.

> 
>>          MSR_K7_EVNTSEL0, MSR_K7_EVNTSEL1, MSR_K7_EVNTSEL2, MSR_K7_EVNTSEL3,
>>          MSR_K7_PERFCTR0, MSR_K7_PERFCTR1, MSR_K7_PERFCTR2, MSR_K7_PERFCTR3,
>>          MSR_F15H_PERF_CTL0, MSR_F15H_PERF_CTL1, MSR_F15H_PERF_CTL2,
>>          MSR_F15H_PERF_CTL3, MSR_F15H_PERF_CTL4, MSR_F15H_PERF_CTL5,
>>          MSR_F15H_PERF_CTR0, MSR_F15H_PERF_CTR1, MSR_F15H_PERF_CTR2,
>>          MSR_F15H_PERF_CTR3, MSR_F15H_PERF_CTR4, MSR_F15H_PERF_CTR5,
> 
> At some point, we may want to consider populating the PMU MSR list
> dynamically, rather than statically enumerating all of them (for both
> AMD and Intel).

The uncertainty of msrs_to_save_all[] may cause troubles for legacy user spaces.
I had draft patches to rewrite pmu msr accesses for host-initiated as first move.

> 
> Reviewed-by: Jim Mattson <jmattson@...gle.com>