When PMU is enabled in QEMU, there is a chance that PMU virtualization is
completely disabled by the KVM module parameter kvm.enable_pmu=N.

The kvm.enable_pmu parameter is introduced since Linux v5.17.
Its permission is 0444. It does not change until a reload of the KVM
module.

Read the kvm.enable_pmu value from the module sysfs to give a chance to
provide more information about vPMU enablement.

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
---
Changed since v2:
  - Rework the code flow following Zhao's suggestion.
  - Return error when:
    (*kvm_enable_pmu == 'N' && X86_CPU(cpu)->enable_pmu)
Changed since v3:
  - Re-split the cases into enable_pmu and !enable_pmu, following Zhao's
    suggestion.
  - Rework the commit messages.
  - Bring back global static variable 'kvm_pmu_disabled' from v2.
Changed since v4:
  - Add Reviewed-by from Zhao.
Changed since v5:
  - Rebase on top of most recent QEMU.
Changed since v6:
  - Add Reviewed-by from Dapeng Mi.

 target/i386/kvm/kvm.c | 61 +++++++++++++++++++++++++++++++------------
 1 file changed, 44 insertions(+), 17 deletions(-)

diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index 577326537e..97782ce070 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -186,6 +186,10 @@ static int has_triple_fault_event;
 static bool has_msr_mcg_ext_ctl;
 
 static int pmu_cap;
+/*
+ * Read from /sys/module/kvm/parameters/enable_pmu.
+ */
+static bool kvm_pmu_disabled;
 
 static struct kvm_cpuid2 *cpuid_cache;
 static struct kvm_cpuid2 *hv_cpuid_cache;
@@ -2067,23 +2071,30 @@ int kvm_arch_pre_create_vcpu(CPUState *cpu, Error **errp)
     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 ((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 (X86_CPU(cpu)->enable_pmu) {
+            if (kvm_pmu_disabled) {
+                warn_report("Failed to enable PMU since "
+                            "KVM's enable_pmu parameter is disabled");
+            }
+        } else {
+            /*
+             * 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 (pmu_cap & KVM_PMU_CAP_DISABLE) {
+                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;
+                }
             }
         }
     }
@@ -3301,6 +3312,7 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
     int ret;
     struct utsname utsname;
     Error *local_err = NULL;
+    g_autofree char *kvm_enable_pmu;
 
     /*
      * Initialize confidential guest (SEV/TDX) context, if required
@@ -3436,6 +3448,21 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
 
     pmu_cap = kvm_check_extension(s, KVM_CAP_PMU_CAPABILITY);
 
+    /*
+     * The enable_pmu parameter is introduced since Linux v5.17,
+     * give a chance to provide more information about vPMU
+     * enablement.
+     *
+     * 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') {
+            kvm_pmu_disabled = true;
+        }
+    }
+
     return 0;
 }
 
-- 
2.39.3