KVM: SVM: Check pmu->version, not enable_pmu, when getting PMC MSRs

Gate access to PMC MSRs based on pmu->version, not on kvm->arch.enable_pmu,
to more accurately reflect KVM's behavior.  This is a glorified nop, as
pmu->version and pmu->nr_arch_gp_counters can only be non-zero if
amd_pmu_refresh() is reached, kvm_pmu_refresh() invokes amd_pmu_refresh()
if and only if kvm->arch.enable_pmu is true, and amd_pmu_refresh() forces
pmu->version to be 1 or 2.

I.e. the following holds true:

  !pmu->nr_arch_gp_counters || kvm->arch.enable_pmu == (pmu->version > 0)

and so the only way for amd_pmu_get_pmc() to return a non-NULL value is if
both kvm->arch.enable_pmu and pmu->version evaluate to true.

No real functional change intended.

Reviewed-by: Sandipan Das <sandipan.das@amd.com>
Tested-by: Xudong Hao <xudong.hao@intel.com>
Link: https://lore.kernel.org/r/20250806195706.1650976-16-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/svm/pmu.c

@@ -41,7 +41,7 @@ static inline struct kvm_pmc *get_gp_pmc_amd(struct kvm_pmu *pmu, u32 msr,
 	struct kvm_vcpu *vcpu = pmu_to_vcpu(pmu);
 	unsigned int idx;
 
-	if (!vcpu->kvm->arch.enable_pmu)
+	if (!pmu->version)
 		return NULL;
 
 	switch (msr) {