Re: [PATCH 1/2] KVM: x86/pmu: Define KVM_PMC_MAX_GENERIC for platform independence

Sean Christopherson
2024-06-20T16:16:43Z

On Thu, Jun 20, 2024, Dapeng Mi wrote:
> The existing macro, KVM_INTEL_PMC_MAX_GENERIC, ambiguously represents the
> maximum supported General Purpose (GP) counter number for both Intel and
> AMD platforms. This could lead to issues if AMD begins to support more GP
> counters than Intel.
>
> To resolve this, a new platform-independent macro, KVM_PMC_MAX_GENERIC,
> is introduced to represent the maximum GP counter number across all x86
> platforms.
>
> No logic changes are introduced in this patch.
>
> Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
> ---
> arch/x86/include/asm/kvm_host.h | 9 +++++----
> arch/x86/kvm/svm/pmu.c | 2 +-
> arch/x86/kvm/vmx/pmu_intel.c | 2 ++
> 3 files changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 57440bda4dc4..18137be6504a 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -534,11 +534,12 @@ struct kvm_pmc {
>
> /* More counters may conflict with other existing Architectural MSRs */
> #define KVM_INTEL_PMC_MAX_GENERIC 8
> -#define MSR_ARCH_PERFMON_PERFCTR_MAX (MSR_ARCH_PERFMON_PERFCTR0 + KVM_INTEL_PMC_MAX_GENERIC - 1)
> -#define MSR_ARCH_PERFMON_EVENTSEL_MAX (MSR_ARCH_PERFMON_EVENTSEL0 + KVM_INTEL_PMC_MAX_GENERIC - 1)
> +#define KVM_AMD_PMC_MAX_GENERIC 6
> +#define KVM_PMC_MAX_GENERIC KVM_INTEL_PMC_MAX_GENERIC

Since we're changing the macro, maybe take the opportunity to use a better name?
E.g. KVM_MAX_NR_GP_COUNTERS? And then in a follow-up patch, give fixed counters
the same treatment, e.g. KVM_MAX_NR_FIXED_COUNTERS. Or maybe KVM_MAX_NR_GP_PMCS
and KVM_MAX_NR_FIXED_PMCS?

> +#define MSR_ARCH_PERFMON_PERFCTR_MAX (MSR_ARCH_PERFMON_PERFCTR0 + KVM_PMC_MAX_GENERIC - 1)
> +#define MSR_ARCH_PERFMON_EVENTSEL_MAX (MSR_ARCH_PERFMON_EVENTSEL0 + KVM_PMC_MAX_GENERIC - 1)

And I'm very, very tempted to say we should simply delete these two, along with
MSR_ARCH_PERFMON_FIXED_CTR_MAX, and just open code the "end" MSR in the one user.
Especially since "KVM" doesn't appear anyone in the name, i.e. because the names
misrepresent KVM's semi-arbitrary max as the *architectural* max.

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 6ad19d913d31..547dfe40d017 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7432,17 +7432,20 @@ static void kvm_probe_msr_to_save(u32 msr_index)
intel_pt_validate_hw_cap(PT_CAP_num_address_ranges) * 2))
return;
break;
- case MSR_ARCH_PERFMON_PERFCTR0 ... MSR_ARCH_PERFMON_PERFCTR_MAX:
+ case MSR_ARCH_PERFMON_PERFCTR0 ...
+ MSR_ARCH_PERFMON_PERFCTR0 + KVM_MAX_NR_GP_COUNTERS - 1:
if (msr_index - MSR_ARCH_PERFMON_PERFCTR0 >=
kvm_pmu_cap.num_counters_gp)
return;
break;
- case MSR_ARCH_PERFMON_EVENTSEL0 ... MSR_ARCH_PERFMON_EVENTSEL_MAX:
+ case MSR_ARCH_PERFMON_EVENTSEL0 ...
+ MSR_ARCH_PERFMON_EVENTSEL0 + KVM_MAX_NR_GP_COUNTERS - 1:
if (msr_index - MSR_ARCH_PERFMON_EVENTSEL0 >=
kvm_pmu_cap.num_counters_gp)
return;
break;
- case MSR_ARCH_PERFMON_FIXED_CTR0 ... MSR_ARCH_PERFMON_FIXED_CTR_MAX:
+ case MSR_ARCH_PERFMON_FIXED_CTR0 ...
+ MSR_ARCH_PERFMON_FIXED_CTR0 + KVM_MAR_NR_FIXED_COUNTERS - 1:
if (msr_index - MSR_ARCH_PERFMON_FIXED_CTR0 >=
kvm_pmu_cap.num_counters_fixed)
return;

> #define KVM_PMC_MAX_FIXED 3
> #define MSR_ARCH_PERFMON_FIXED_CTR_MAX (MSR_ARCH_PERFMON_FIXED_CTR0 + KVM_PMC_MAX_FIXED - 1)
> -#define KVM_AMD_PMC_MAX_GENERIC 6
>
> struct kvm_pmu {
> u8 version;
> @@ -554,7 +555,7 @@ struct kvm_pmu {
> u64 global_status_rsvd;
> u64 reserved_bits;
> u64 raw_event_mask;
> - struct kvm_pmc gp_counters[KVM_INTEL_PMC_MAX_GENERIC];
> + struct kvm_pmc gp_counters[KVM_PMC_MAX_GENERIC];
> struct kvm_pmc fixed_counters[KVM_PMC_MAX_FIXED];
>
> /*
> diff --git a/arch/x86/kvm/svm/pmu.c b/arch/x86/kvm/svm/pmu.c
> index 6e908bdc3310..2fca247798eb 100644
> --- a/arch/x86/kvm/svm/pmu.c
> +++ b/arch/x86/kvm/svm/pmu.c
> @@ -218,7 +218,7 @@ static void amd_pmu_init(struct kvm_vcpu *vcpu)
> int i;
>
> BUILD_BUG_ON(KVM_AMD_PMC_MAX_GENERIC > AMD64_NUM_COUNTERS_CORE);
> - BUILD_BUG_ON(KVM_AMD_PMC_MAX_GENERIC > INTEL_PMC_MAX_GENERIC);
> + BUILD_BUG_ON(KVM_AMD_PMC_MAX_GENERIC > KVM_PMC_MAX_GENERIC);
>
> for (i = 0; i < KVM_AMD_PMC_MAX_GENERIC ; i++) {
> pmu->gp_counters[i].type = KVM_PMC_GP;
> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> index fb5cbd6cbeff..a4b0bee04596 100644
> --- a/arch/x86/kvm/vmx/pmu_intel.c
> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> @@ -570,6 +570,8 @@ static void intel_pmu_init(struct kvm_vcpu *vcpu)
> struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
> struct lbr_desc *lbr_desc = vcpu_to_lbr_desc(vcpu);
>
> + BUILD_BUG_ON(KVM_INTEL_PMC_MAX_GENERIC > KVM_PMC_MAX_GENERIC);

Rather than BUILD_BUG_ON() for both Intel and AMD, can't we just do?

#define KVM_MAX_NR_GP_COUNTERS max(KVM_INTEL_PMC_MAX_GENERIC, KVM_AMD_PMC_MAX_GENERIC)

> +
> for (i = 0; i < KVM_INTEL_PMC_MAX_GENERIC; i++) {
> pmu->gp_counters[i].type = KVM_PMC_GP;
> pmu->gp_counters[i].vcpu = vcpu;
> --
> 2.34.1
>