[PATCH] target/arm: Enable KVM PMU partitioning and counter limit

[Colton Lewis <coltonlewis@google.com>]

Add CPU properties to configure KVM ARM64 PMU counter partitioning:
- 'pmu-partition' (bool): enables KVM_ARM_VCPU_PMU_V3_ENABLE_PARTITION
- 'pmu-num-counters' (uint32): sets KVM_ARM_VCPU_PMU_V3_SET_NR_COUNTERS

When 'pmu-partition=on' and 'pmu-num-counters=N' are passed via -cpu,
QEMU enables hardware PMU partitioning and reserves N general-purpose
event counters for the guest prior to calling KVM_ARM_VCPU_PMU_V3_INIT.

Note that this patch is an example to demonstrate the KVM ARM64 PMU
Partitioning uAPI [1]. That is why it aborts immediately if setting the
device attributes does not succeed.

[1] https://lore.kernel.org/kvmarm/20260924172928.2110956-1-coltonlewis@google.com/

Signed-off-by: Colton Lewis <coltonlewis@google.com>
---
 target/arm/cpu.c | 20 ++++++++++++++++++++
 target/arm/cpu.h |  4 ++++
 target/arm/kvm.c | 32 ++++++++++++++++++++++++++++++++
 3 files changed, 56 insertions(+)

diff --git a/target/arm/cpu.c b/target/arm/cpu.c
index f58a1db843..7538c6a24c 100644
--- a/target/arm/cpu.c
+++ b/target/arm/cpu.c
@@ -1359,6 +1359,20 @@ static bool arm_get_pmu(Object *obj, Error **errp)
     return cpu->has_pmu;
 }
 
+static bool arm_get_pmu_partition(Object *obj, Error **errp)
+{
+    ARMCPU *cpu = ARM_CPU(obj);
+
+    return cpu->kvm_pmu_partition;
+}
+
+static void arm_set_pmu_partition(Object *obj, bool value, Error **errp)
+{
+    ARMCPU *cpu = ARM_CPU(obj);
+
+    cpu->kvm_pmu_partition = value;
+}
+
 static void arm_set_pmu(Object *obj, bool value, Error **errp)
 {
     ARMCPU *cpu = ARM_CPU(obj);
@@ -1604,6 +1618,12 @@ static void arm_cpu_post_init(Object *obj)
     if (arm_feature(&cpu->env, ARM_FEATURE_PMU)) {
         cpu->has_pmu = true;
         object_property_add_bool(obj, "pmu", arm_get_pmu, arm_set_pmu);
+        object_property_add_bool(obj, "pmu-partition",
+                                 arm_get_pmu_partition,
+                                 arm_set_pmu_partition);
+        object_property_add_uint32_ptr(obj, "pmu-num-counters",
+                                       &cpu->kvm_pmu_nr_counters,
+                                       OBJ_PROP_FLAG_READWRITE);
     }
 
     /*
diff --git a/target/arm/cpu.h b/target/arm/cpu.h
index e3f931dba2..1320e76fd4 100644
--- a/target/arm/cpu.h
+++ b/target/arm/cpu.h
@@ -1004,6 +1004,10 @@ struct ArchCPU {
     bool has_el3;
     /* CPU has PMU (Performance Monitor Unit) */
     bool has_pmu;
+    /* CPU has PMU Partitioning enabled in KVM mode */
+    bool kvm_pmu_partition;
+    /* Number of PMU counters allocated to guest in KVM mode */
+    uint32_t kvm_pmu_nr_counters;
     /* CPU has VFP */
     bool has_vfp;
     /* CPU has 32 VFP registers */
diff --git a/target/arm/kvm.c b/target/arm/kvm.c
index ed99be7fd8..3aa8243278 100644
--- a/target/arm/kvm.c
+++ b/target/arm/kvm.c
@@ -1846,6 +1846,10 @@ static bool kvm_arm_set_device_attr(ARMCPU *cpu, struct kvm_device_attr *attr,
     return true;
 }
 
+#ifndef KVM_ARM_VCPU_PMU_V3_ENABLE_PARTITION
+#define KVM_ARM_VCPU_PMU_V3_ENABLE_PARTITION 5
+#endif
+
 void kvm_arm_pmu_init(ARMCPU *cpu)
 {
     struct kvm_device_attr attr = {
@@ -1856,6 +1860,34 @@ void kvm_arm_pmu_init(ARMCPU *cpu)
     if (!cpu->has_pmu) {
         return;
     }
+
+    if (cpu->kvm_pmu_partition) {
+        uint32_t partition = 1;
+        struct kvm_device_attr part_attr = {
+            .group = KVM_ARM_VCPU_PMU_V3_CTRL,
+            .attr = KVM_ARM_VCPU_PMU_V3_ENABLE_PARTITION,
+            .addr = (intptr_t)&partition,
+        };
+
+        if (!kvm_arm_set_device_attr(cpu, &part_attr, "PMU partition")) {
+            error_report("failed to enable KVM PMU partition");
+            abort();
+        }
+    }
+
+    if (cpu->kvm_pmu_nr_counters > 0) {
+        struct kvm_device_attr cnt_attr = {
+            .group = KVM_ARM_VCPU_PMU_V3_CTRL,
+            .attr = KVM_ARM_VCPU_PMU_V3_SET_NR_COUNTERS,
+            .addr = (intptr_t)&cpu->kvm_pmu_nr_counters,
+        };
+
+        if (!kvm_arm_set_device_attr(cpu, &cnt_attr, "PMU nr_counters")) {
+            error_report("failed to set KVM PMU nr_counters");
+            abort();
+        }
+    }
+
     if (!kvm_arm_set_device_attr(cpu, &attr, "PMU")) {
         error_report("failed to init PMU");
         abort();

base-commit: 014b1f730c0faa08067f8e9b5829e62ea3d4b177
-- 
2.56.0.rc1.315.gc6ed9934b7-goog