[v2,04/10] target/i386/kvm: set KVM_PMU_CAP_DISABLE if "-pmu" is configured

Commit Message

Dongli Zhang March 2, 2025, 10 p.m. UTC

Although AMD PERFCORE and PerfMonV2 are removed when "-pmu" is configured,
there is no way to fully disable KVM AMD PMU virtualization. Neither
"-cpu host,-pmu" nor "-cpu EPYC" achieves this.

As a result, the following message still appears in the VM dmesg:

[    0.263615] Performance Events: AMD PMU driver.

However, the expected output should be:

[    0.596381] Performance Events: PMU not available due to virtualization, using software events only.
[    0.600972] NMI watchdog: Perf NMI watchdog permanently disabled

This occurs because AMD does not use any CPUID bit to indicate PMU
availability.

To address this, KVM_CAP_PMU_CAPABILITY is used to set KVM_PMU_CAP_DISABLE
when "-pmu" is configured.

Signed-off-by: Dongli Zhang <[e-mail]>
---
Changed since v1:
  - Switch back to the initial implementation with "-pmu".
https://lore.kernel.org/all/[e-mail]
  - Mention that "KVM_PMU_CAP_DISABLE doesn't change the PMU behavior on
    Intel platform because current "pmu" property works as expected."

 target/i386/kvm/kvm.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

Patch

diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index f41e190fb8..5c8a852dbd 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -176,6 +176,8 @@  static int has_triple_fault_event;
 
 static bool has_msr_mcg_ext_ctl;
 
+static int has_pmu_cap;
+
 static struct kvm_cpuid2 *cpuid_cache;
 static struct kvm_cpuid2 *hv_cpuid_cache;
 static struct kvm_msr_list *kvm_feature_msrs;
@@ -2053,6 +2055,33 @@  full:
 
 int kvm_arch_pre_create_vcpu(CPUState *cpu, Error **errp)
 {
+    static bool first = true;
+    int ret;
+
+    if (first) {
+        first = false;
+
+        /*
+         * Since Linux v5.18, KVM provides a VM-level capability to easily
+         * disable PMUs; however, QEMU has been providing PMU property per
+         * CPU since v1.6. In order to accommodate both, have to configure
+         * the VM-level capability here.
+         *
+         * KVM_PMU_CAP_DISABLE doesn't change the PMU
+         * behavior on Intel platform because current "pmu" property works
+         * as expected.
+         */
+        if (has_pmu_cap && !X86_CPU(cpu)->enable_pmu) {
+            ret = kvm_vm_enable_cap(kvm_state, KVM_CAP_PMU_CAPABILITY, 0,
+                                    KVM_PMU_CAP_DISABLE);
+            if (ret < 0) {
+                error_setg_errno(errp, -ret,
+                                 "Failed to set KVM_PMU_CAP_DISABLE");
+                return ret;
+            }
+        }
+    }
+
     return 0;
 }
 
@@ -3351,6 +3380,8 @@  int kvm_arch_init(MachineState *ms, KVMState *s)
         }
     }
 
+    has_pmu_cap = kvm_check_extension(s, KVM_CAP_PMU_CAPABILITY);
+
     return 0;
 }