On 4/8/2022 3:26 am, Sean Christopherson wrote:
Explicitly check for the absence of host support for LBRs or PEBS when
userspace attempts to enable said features by writing PERF_CAPABILITIES.
Comparing host support against the incoming value is unnecessary and
weird since the checks are buried inside an if-statement that verifies
userspace wants to enable the feature.
If you mean this part in the KVM:
case MSR_IA32_PERF_CAPABILITIES: {
...
if (data & ~msr_ent.data)
return 1;
...
then this patch brings a flaw, for example:
a user space can successfully set 0x1 on a host that reports a value of 0x5,
which should not happen since the semantics of 0x1 and 0x5 for LBR_FMT
may be completely different from the guest LBR driver's perspective.
For such a model-specific feature, it needs to write to PERF_CAPABILITIES
the exact value reported by the host/kvm.
A selftest is proposed in the hope of guarding this contract.
No functional change intended.
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
arch/x86/kvm/vmx/vmx.c | 6 ++----
1 file changed, 2 insertions(+), 4 deletions(-)
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index d7f8331d6f7e..0ada0ee234b7 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2323,15 +2323,13 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
if (data && !vcpu_to_pmu(vcpu)->version)
return 1;
if (data & PMU_CAP_LBR_FMT) {
- if ((data & PMU_CAP_LBR_FMT) !=
- (vmx_get_perf_capabilities() & PMU_CAP_LBR_FMT))
+ if (!(vmx_get_perf_capabilities() & PMU_CAP_LBR_FMT))
return 1;
if (!cpuid_model_is_consistent(vcpu))
return 1;
}
if (data & PERF_CAP_PEBS_FORMAT) {
- if ((data & PERF_CAP_PEBS_MASK) !=
- (vmx_get_perf_capabilities() & PERF_CAP_PEBS_MASK))
+ if (!(vmx_get_perf_capabilities() & PERF_CAP_PEBS_MASK))
return 1;
if (!guest_cpuid_has(vcpu, X86_FEATURE_DS))
return 1;
