[PATCH 1/2] KVM: x86/pmu: Reset perf_capabilities in vcpu to 0 if PDCM is disabled

[Mingwei Zhang]

Reset vcpu->arch.perf_capabilities to 0 if PDCM is disabled in guest cpuid.
Without this, there is an issue in live migration. In particular, to
migrate a VM with no PDCM enabled, VMM on the source is able to retrieve a
non-zero value by reading the MSR_IA32_PERF_CAPABILITIES. However, VMM on
the target is unable to set the value. This creates confusions on the user
side.

Fundamentally, it is because vcpu->arch.perf_capabilities as the cached
value of MSR_IA32_PERF_CAPABILITIES is incorrect, and there is nothing
wrong on the kvm_get_msr_common() which just reads
vcpu->arch.perf_capabilities.

Fix the issue by adding the reset code in kvm_vcpu_after_set_cpuid(), i.e.
early in VM setup time.

Cc: Aaron Lewis <aaronlewis@xxxxxxxxxx>
Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
---
 arch/x86/kvm/cpuid.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index adba49afb5fe..416bee03c42a 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -369,6 +369,9 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 	vcpu->arch.maxphyaddr = cpuid_query_maxphyaddr(vcpu);
 	vcpu->arch.reserved_gpa_bits = kvm_vcpu_reserved_gpa_bits_raw(vcpu);
 
+	/* Reset MSR_IA32_PERF_CAPABILITIES guest value to 0 if PDCM is off. */
+	if (!guest_cpuid_has(vcpu, X86_FEATURE_PDCM))
+		vcpu->arch.perf_capabilities = 0;
 	kvm_pmu_refresh(vcpu);
 	vcpu->arch.cr4_guest_rsvd_bits =
 	    __cr4_reserved_bits(guest_cpuid_has, vcpu);
-- 
2.43.0.429.g432eaa2c6b-goog