There is no way to distinguish between the following scenarios:

(1) KVM_CAP_PMU_CAPABILITY is not supported.
(2) KVM_CAP_PMU_CAPABILITY is supported but disabled via the module
parameter kvm.enable_pmu=N.

In scenario (1), there is no way to fully disable AMD PMU virtualization.

In scenario (2), PMU virtualization is completely disabled by the KVM
module.

To help determine the scenario, read the kvm.enable_pmu value from the
sysfs module parameter.

There isn't any requirement to initialize 'pmu_version',
'num_pmu_gp_counters' or 'num_pmu_fixed_counters', if kvm.enable_pmu=N.

In addition, return error when kvm.enable_pmu=N but the user wants to enable
vPMU.

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
---
Changed since v2:
  - Rework the code flow following Zhao's suggestion.
  - Return error when:
    (*kvm_enable_pmu == 'N' && X86_CPU(cpu)->enable_pmu)

 target/i386/kvm/kvm.c | 36 +++++++++++++++++++++++++++++-------
 1 file changed, 29 insertions(+), 7 deletions(-)

diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index 6b49549f1b..f68d5a0578 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -2051,13 +2051,35 @@ int kvm_arch_pre_create_vcpu(CPUState *cpu, Error **errp)
          * behavior on Intel platform because current "pmu" property works
          * as expected.
          */
-        if ((pmu_cap & KVM_PMU_CAP_DISABLE) && !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;
+        if (pmu_cap) {
+            if ((pmu_cap & KVM_PMU_CAP_DISABLE) &&
+                !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;
+                }
+            }
+        } else {
+            /*
+             * KVM_CAP_PMU_CAPABILITY is introduced in Linux v5.18. For old
+             * linux, we have to check enable_pmu parameter for vPMU support.
+             */
+            g_autofree char *kvm_enable_pmu;
+
+            /*
+             * The kvm.enable_pmu's permission is 0444. It does not change until
+             * a reload of the KVM module.
+             */
+            if (g_file_get_contents("/sys/module/kvm/parameters/enable_pmu",
+                                    &kvm_enable_pmu, NULL, NULL)) {
+                if (*kvm_enable_pmu == 'N' && X86_CPU(cpu)->enable_pmu) {
+                    error_setg(errp, "Failed to enable PMU since "
+                               "KVM's enable_pmu parameter is disabled");
+                    return -EPERM;
+                }
             }
         }
     }
-- 
2.39.3