Re: [PATCH v3 6/8] KVM: x86/svm/pmu: Add AMD PerfMonV2 support

From: Like Xu <like.xu.linux@gmail.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Sandipan Das <sandipan.das@amd.com>,
	Jim Mattson <jmattson@google.com>
Subject: Re: [PATCH v3 6/8] KVM: x86/svm/pmu: Add AMD PerfMonV2 support
Date: Mon, 6 Feb 2023 15:53:00 +0800	[thread overview]
Message-ID: <c7e0a5b1-0fd0-e2b5-20ca-fc86a1d883db@gmail.com> (raw)
In-Reply-To: <Y9BzYzEjAwUA+wuy@google.com>

On 25/1/2023 8:10 am, Sean Christopherson wrote:
> On Fri, Nov 11, 2022, Like Xu wrote:
> On Fri, Nov 11, 2022, Like Xu wrote:
>> @@ -162,20 +179,42 @@ static int amd_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>   static void amd_pmu_refresh(struct kvm_vcpu *vcpu)
>>   {
>>        struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
>> +     struct kvm_cpuid_entry2 *entry;
>> +     union cpuid_0x80000022_ebx ebx;
>>
>> -     if (guest_cpuid_has(vcpu, X86_FEATURE_PERFCTR_CORE))
>> -             pmu->nr_arch_gp_counters = AMD64_NUM_COUNTERS_CORE;
>> +     pmu->version = 1;
>> +     if (kvm_cpu_cap_has(X86_FEATURE_AMD_PMU_V2) &&
> 
> Why check kvm_cpu_cap support?  I.e. what will go wrong if userspace enumerates
> PMU v2 to the guest without proper hardware/KVM support.
> 
> If this is _necessary_ to protect the host kernel, then we should probably have
> a helper to query PMU features, e.g.
> 
> static __always_inline bool guest_pmu_has(struct kvm_vcpu *vcpu,
>                                            unsigned int x86_feature)
> {
>          return kvm_cpu_cap_has(x86_feature) &&
>                 guest_cpuid_has(vcpu, x86_feature);
> }
> 
> 
> 
>> +         guest_cpuid_has(vcpu, X86_FEATURE_AMD_PMU_V2)) {
>> +             pmu->version = 2;
>> +             entry = kvm_find_cpuid_entry_index(vcpu, 0x80000022, 0);
>> +             ebx.full = entry->ebx;
>> +             pmu->nr_arch_gp_counters = min3((unsigned int)ebx.split.num_core_pmc,
>> +                                             (unsigned int)kvm_pmu_cap.num_counters_gp,
>> +                                             (unsigned int)KVM_AMD_PMC_MAX_GENERIC);
> 
> Blech.  This really shouldn't be necessary, KVM should tweak kvm_pmu_cap.num_counters_gp
> as needed during initialization to ensure num_counters_gp doesn't exceed KVM's
> internal limits.
> 
> Posted a patch[*], please take a look.  As mentioned in that thread, I'll somewhat
> speculatively apply that series sooner than later so that you can use it a base
> for this series (assuming the patch isn't busted).
> 
> [*] https://lore.kernel.org/all/20230124234905.3774678-2-seanjc@google.com
> 
>> +     }
>> +
>> +     /* Commitment to minimal PMCs, regardless of CPUID.80000022 */
> 
> Please expand this comment.  I'm still not entirely sure I've interpreted it correctly,
> and I'm not sure that I agree with the code.

In the first version [1], I used almost the same if-elif-else sequence
but the concerns from JimM[2] has changed my mind:

"Nonetheless, for compatibility with old software, Fn8000_0022_EBX can never
report less than four counters (or six, if Fn8000_0001_ECX[PerfCtrExtCore] is set)."

Both in amd_pmu_refresh() and in __do_cpuid_func(), KVM implements
this using the override approach of first applying the semantics of
AMD_PMU_V2 and then implementing a minimum number of counters
supported based on whether or not guest have  PERFCTR_CORE,
the proposed if-elif-else does not fulfill this need.

[1] 20220905123946.95223-4-likexu@tencent.com/
[2] CALMp9eQObuiJGV=YrAU9Fw+KoXfJtZMJ-KUs-qCOVd+R9zGBpw@mail.gmail.com

> 
>> +     if (kvm_cpu_cap_has(X86_FEATURE_PERFCTR_CORE) &&
> 
> AFAICT, checking kvm_cpu_cap_has() is an unrelated change.  Either it's a bug fix
> and belongs in a separate patch, or it's unnecessary and should be dropped.
> 
>> +         guest_cpuid_has(vcpu, X86_FEATURE_PERFCTR_CORE))
>> +             pmu->nr_arch_gp_counters = max_t(unsigned int,
>> +                                              pmu->nr_arch_gp_counters,
>> +                                              AMD64_NUM_COUNTERS_CORE);
>>        else
>> -             pmu->nr_arch_gp_counters = AMD64_NUM_COUNTERS;
>> +             pmu->nr_arch_gp_counters = max_t(unsigned int,
>> +                                              pmu->nr_arch_gp_counters,
>> +                                              AMD64_NUM_COUNTERS);
> 
> Using max() doesn't look right.  E.g. if KVM ends up running on some odd setup
> where ebx.split.num_core_pmc/kvm_pmu_cap.num_counters_gp is less than
> AMD64_NUM_COUNTERS_CORE or AMD64_NUM_COUNTERS.
> 
> Or more likely, if userspace says "only expose N counters to this guest".
> 
> Shouldn't this be something like?
> 
> 	if (guest_cpuid_has(vcpu, X86_FEATURE_AMD_PMU_V2))
> 		pmu->nr_arch_gp_counters = min(ebx.split.num_core_pmc,
> 					       kvm_pmu_cap.num_counters_gp);
> 	else if (guest_cpuid_has(vcpu, X86_FEATURE_PERFCTR_CORE))
> 		pmu->nr_arch_gp_counters = AMD64_NUM_COUNTERS_CORE;
> 	else
> 		pmu->nr_arch_gp_counters = AMD64_NUM_COUNTERSE;
> 