[PATCH v3 11/18] KVM: x86/pmu: Emulate IA32_PERF_GLOBAL_STATUS_SET MSR

[Zide Chen]

Intel PerfMon v4 introduces IA32_PERF_GLOBAL_STATUS_SET (0x391) to
allow software to set individual bits in the global status MSR. Reads
of IA32_PERF_GLOBAL_STATUS_SET always return zero.

IA32_PERF_GLOBAL_STATUS_SET is not part of the VM state, so it doesn't
need to be advertised to userspace, nor saved and restored during live
migration.

Always intercept IA32_PERF_GLOBAL_STATUS_SET to prevent the guest from
modifying status bits that are not virtualized by KVM.

Bit 63 (CondChgd) stays unreserved in pmu->global_status_rsvd to
architecturally allow the guest to write this bit on STATUS_RESET (0x390)
to clear it, and allow userspace to set it on GLOBAL_STATUS (0x38E).

However, bit 63 is architecturally reserved in STATUS_SET per the SDM.
Reject writes to bit 63, injecting #GP for guest writes. While older
processors had silicon errata that accepted writes without #GP (e.g.
SKX95), strictly enforce the architectural specification for
deterministic behavior across migrations.

Platform-specific EDS clarifies that CondChgd indicates a change to
IA32_MISC_ENABLE[7], which KVM intercepts and keeps strictly read-only
for guests. This guarantees hardware will never assert CondChgd during
guest execution.

Thus, the architectural asymmetry among these three MSRs is safe; being
unable to restore bit 63 to hardware via STATUS_SET is benign because
the guest will never legitimately observe CondChgd. Filter it out
defensively before writing to hardware in intel_mediated_pmu_load().

Originally-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
Signed-off-by: Zide Chen <zide.chen@xxxxxxxxx>
---
v3:
- Split from combined v2 patch into standalone GLOBAL_STATUS_SET patch.
- Always intercept IA32_PERF_GLOBAL_STATUS_SET to prevent passthrough of
  unvirtualized status bits. (Jim)
- Reject writing bit 63 (CondChgd) to IA32_PERF_GLOBAL_STATUS_SET. (Jim)
- Filter out bit 63 before writing to hardware in intel_mediated_pmu_load().
- Move validation for MSR_CORE_PERF_GLOBAL_STATUS_SET to pmu_intel.c rather
  than common pmu.c to avoid exposing the Intel MSR on AMD.

v2:
- Add the missing vmcs02 updates for these two MSRs.
- Change "> 3" to ">= 4" to make the "v4-gated" more obvious and match
  the existing code style.
---
 arch/x86/kvm/pmu.c           | 13 +++++++++++++
 arch/x86/kvm/vmx/pmu_intel.c | 14 ++++++++++++--
 2 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index 27d6a9169821..2cb5bccc7044 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -855,6 +855,7 @@ int kvm_pmu_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 	case MSR_AMD64_PERF_CNTR_GLOBAL_STATUS_CLR:
 	case MSR_AMD64_PERF_CNTR_GLOBAL_STATUS_SET:
 	case MSR_CORE_PERF_GLOBAL_OVF_CTRL:
+	case MSR_CORE_PERF_GLOBAL_STATUS_SET:
 		msr_info->data = 0;
 		break;
 	default:
@@ -921,6 +922,18 @@ int kvm_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		if (!msr_info->host_initiated)
 			pmu->global_status &= ~data;
 		break;
+	case MSR_CORE_PERF_GLOBAL_STATUS_SET:
+		/*
+		 * Bit 63 (CondChgd) is architecturally reserved in STATUS_SET.
+		 * Older CPUs errantly accepted writes without #GP (e.g. SKX95),
+		 * but KVM strictly enforces the architectural spec to ensure
+		 * consistent behavior across migrations.
+		 */
+		if (data & (pmu->global_status_rsvd | GLOBAL_STATUS_COND_CHG))
+			return 1;
+		if (!msr_info->host_initiated)
+			pmu->global_status |= data;
+		break;
 	case MSR_AMD64_PERF_CNTR_GLOBAL_STATUS_SET:
 		if (!msr_info->host_initiated)
 			pmu->global_status |= data & ~pmu->global_status_rsvd;
diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
index ce668ea08da5..e81a3f8b9b02 100644
--- a/arch/x86/kvm/vmx/pmu_intel.c
+++ b/arch/x86/kvm/vmx/pmu_intel.c
@@ -197,6 +197,8 @@ static bool intel_is_valid_msr(struct kvm_vcpu *vcpu, u32 msr)
 	case MSR_CORE_PERF_GLOBAL_OVF_CTRL:
 	case MSR_CORE_PERF_FIXED_CTR_CTRL:
 		return kvm_pmu_has_perf_global_ctrl(pmu);
+	case MSR_CORE_PERF_GLOBAL_STATUS_SET:
+		return pmu->version >= 4;
 	case MSR_IA32_PEBS_ENABLE:
 		ret = vcpu_get_perf_capabilities(vcpu) & PERF_CAP_PEBS_FORMAT;
 		break;
@@ -827,8 +829,16 @@ static void intel_mediated_pmu_load(struct kvm_vcpu *vcpu)
 	toggle = pmu->global_status ^ global_status;
 	if (global_status & toggle)
 		wrmsrq(MSR_CORE_PERF_GLOBAL_OVF_CTRL, global_status & toggle);
-	if (pmu->global_status & toggle)
-		wrmsrq(MSR_CORE_PERF_GLOBAL_STATUS_SET, pmu->global_status & toggle);
+
+	/*
+	 * CondChgd is architecturally reserved in GLOBAL_STATUS_SET. Unless
+	 * set by malicious userspace, this bit should never be set in
+	 * pmu->global_status; filter it out defensively to protect against
+	 * a host #GP.
+	 */
+	if ((pmu->global_status & toggle) & ~GLOBAL_STATUS_COND_CHG)
+		wrmsrq(MSR_CORE_PERF_GLOBAL_STATUS_SET,
+		       (pmu->global_status & toggle) & ~GLOBAL_STATUS_COND_CHG);
 
 	wrmsrq(MSR_CORE_PERF_FIXED_CTR_CTRL, pmu->fixed_ctr_ctrl_hw);
 }
-- 
2.56.0