[PATCH v8 09/11] KVM: arm64: PMU: Implement fixed-counters-only emulation
Akihiko Odaki
odaki at rsg.ci.i.u-tokyo.ac.jp
Fri Jul 10 04:15:03 PDT 2026
Add internal state for PMUv3 emulation without programmable event
counters. When fixed-counters-only mode is active, KVM reports no
programmable counters and hides PMCEID, avoiding event-counter state
whose behavior can depend on the selected hardware PMU.
The cycle counter still uses a host perf event. Unlike the normal PMU
path, fixed-counters-only mode may create that event from the hardware
PMU attached to the VCPU's current pCPU. If the VCPU later loads on a
pCPU that is not covered by the existing event's PMU, request a PMU
reload so the cycle counter can be recreated against the new pCPU's PMU.
Keep this affinity check limited to fixed-counters-only VMs; the normal
programmable-counter mode continues to use the VM-wide PMU and does not
need per-load reload decisions.
Add a separate internal flag for explicit userspace PMU selection. The
UAPI wiring added later will use it to keep explicit PMU selection and
fixed-counters-only mode mutually exclusive while still allowing
fixed-counters-only mode to replace the default PMU selected during
KVM_ARM_VCPU_INIT.
The UAPI wiring that sets the fixed-counters-only flag and records
explicit PMU selection is added later in the series.
Assisted-by: Codex:gpt-5.5
Signed-off-by: Akihiko Odaki <odaki at rsg.ci.i.u-tokyo.ac.jp>
---
arch/arm64/include/asm/kvm_host.h | 4 +++
arch/arm64/kvm/arm.c | 2 ++
arch/arm64/kvm/pmu-emul.c | 69 ++++++++++++++++++++++++++++++++++-----
include/kvm/arm_pmu.h | 2 ++
4 files changed, 69 insertions(+), 8 deletions(-)
diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 0c39d9db7d57..aa07b05b8231 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -353,6 +353,10 @@ struct kvm_arch {
#define KVM_ARCH_FLAG_WRITABLE_IMP_ID_REGS 10
/* Unhandled SEAs are taken to userspace */
#define KVM_ARCH_FLAG_EXIT_SEA 11
+ /* PMUv3 is emulated with an explicitly specified hardware PMU */
+#define KVM_ARCH_FLAG_PMU_V3_EXPLICIT 12
+ /* PMUv3 is emulated without progammable event counters */
+#define KVM_ARCH_FLAG_PMU_V3_FIXED_COUNTERS_ONLY 13
unsigned long flags;
/* VM-wide vCPU feature set */
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 58d0783f254d..b21bd8cf13c9 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -638,6 +638,7 @@ static bool kvm_vcpu_should_clear_twe(struct kvm_vcpu *vcpu)
void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
{
struct kvm_s2_mmu *mmu;
+ int last_cpu = vcpu->cpu;
int *last_ran;
if (is_protected_kvm_enabled())
@@ -687,6 +688,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
if (has_vhe())
kvm_vcpu_load_vhe(vcpu);
kvm_arch_vcpu_load_fp(vcpu);
+ kvm_vcpu_load_pmu(vcpu, last_cpu);
kvm_vcpu_pmu_restore_guest(vcpu);
if (kvm_arm_is_pvtime_enabled(&vcpu->arch))
kvm_make_request(KVM_REQ_RECORD_STEAL, vcpu);
diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
index 1dd8aa6a027d..1aa115aee781 100644
--- a/arch/arm64/kvm/pmu-emul.c
+++ b/arch/arm64/kvm/pmu-emul.c
@@ -83,6 +83,11 @@ u64 kvm_pmu_evtyper_mask(struct kvm *kvm)
return mask;
}
+static bool kvm_pmu_fixed_counters_only(struct kvm *kvm)
+{
+ return test_bit(KVM_ARCH_FLAG_PMU_V3_FIXED_COUNTERS_ONLY, &kvm->arch.flags);
+}
+
/**
* kvm_pmc_is_64bit - determine if counter is 64bit
* @pmc: counter context
@@ -330,7 +335,12 @@ u64 kvm_pmu_implemented_counter_mask(struct kvm_vcpu *vcpu)
static void kvm_pmc_enable_perf_event(struct kvm_pmc *pmc)
{
- if (!pmc->perf_event) {
+ struct kvm_vcpu *vcpu = kvm_pmc_to_vcpu(pmc);
+
+ if (!pmc->perf_event ||
+ (kvm_pmu_fixed_counters_only(vcpu->kvm) &&
+ !cpumask_test_cpu(READ_ONCE(vcpu->cpu),
+ &to_arm_pmu(pmc->perf_event->pmu)->supported_cpus))) {
kvm_pmu_create_perf_event(pmc);
return;
}
@@ -694,14 +704,10 @@ static struct arm_pmu *kvm_pmu_probe_armpmu(int cpu)
return NULL;
}
-/**
- * kvm_pmu_create_perf_event - create a perf event for a counter
- * @pmc: Counter context
- */
-static void kvm_pmu_create_perf_event(struct kvm_pmc *pmc)
+static void kvm_pmu_create_perf_event_with_pmu(struct kvm_pmc *pmc,
+ struct arm_pmu *arm_pmu)
{
struct kvm_vcpu *vcpu = kvm_pmc_to_vcpu(pmc);
- struct arm_pmu *arm_pmu = vcpu->kvm->arch.arm_pmu;
struct perf_event *event;
struct perf_event_attr attr;
int eventsel;
@@ -735,7 +741,7 @@ static void kvm_pmu_create_perf_event(struct kvm_pmc *pmc)
* Don't create an event if we're running on hardware that requires
* PMUv3 event translation and we couldn't find a valid mapping.
*/
- eventsel = kvm_map_pmu_event(vcpu->kvm->arch.arm_pmu, eventsel);
+ eventsel = kvm_map_pmu_event(arm_pmu, eventsel);
if (eventsel < 0)
return;
@@ -780,6 +786,29 @@ static void kvm_pmu_create_perf_event(struct kvm_pmc *pmc)
pmc->perf_event = event;
}
+/**
+ * kvm_pmu_create_perf_event - create a perf event for a counter
+ * @pmc: Counter context
+ */
+static void kvm_pmu_create_perf_event(struct kvm_pmc *pmc)
+{
+ struct kvm_vcpu *vcpu = kvm_pmc_to_vcpu(pmc);
+ struct arm_pmu *arm_pmu = vcpu->kvm->arch.arm_pmu;
+
+ if (kvm_pmu_fixed_counters_only(vcpu->kvm)) {
+ do {
+ arm_pmu = kvm_pmu_probe_armpmu(READ_ONCE(vcpu->cpu));
+
+ if (WARN_ON_ONCE(!arm_pmu))
+ return;
+
+ kvm_pmu_create_perf_event_with_pmu(pmc, arm_pmu);
+ } while (!cpumask_test_cpu(READ_ONCE(vcpu->cpu), &arm_pmu->supported_cpus));
+ } else {
+ kvm_pmu_create_perf_event_with_pmu(pmc, arm_pmu);
+ }
+}
+
/**
* kvm_pmu_set_counter_event_type - set selected counter to monitor some event
* @vcpu: The vcpu pointer
@@ -813,6 +842,15 @@ void kvm_host_pmu_init(struct arm_pmu *pmu)
if (!pmuv3_implemented(kvm_arm_pmu_get_pmuver_limit()))
return;
+ /*
+ * IMPDEF PMUv3 traps are non-architectural, and KVM cannot assume a
+ * uniform PMUv3-compatible arm_pmu is available on all CPUs.
+ */
+ if (cpus_have_final_cap(ARM64_WORKAROUND_PMUV3_IMPDEF_TRAPS)) {
+ kvm_info("Non-architectural PMU, tainting kernel\n");
+ add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
+ }
+
guard(mutex)(&arm_pmus_lock);
entry = kmalloc_obj(*entry);
@@ -865,6 +903,9 @@ u64 kvm_pmu_get_pmceid(struct kvm_vcpu *vcpu, bool pmceid1)
u64 val, mask = 0;
int base, i, nr_events;
+ if (kvm_pmu_fixed_counters_only(vcpu->kvm))
+ return 0;
+
if (!pmceid1) {
val = compute_pmceid0(cpu_pmu);
base = 0;
@@ -892,6 +933,15 @@ u64 kvm_pmu_get_pmceid(struct kvm_vcpu *vcpu, bool pmceid1)
return val & mask;
}
+void kvm_vcpu_load_pmu(struct kvm_vcpu *vcpu, int last_cpu)
+{
+ if (!kvm_pmu_fixed_counters_only(vcpu->kvm) || vcpu->cpu == last_cpu || last_cpu == -1)
+ return;
+
+ if (kvm_pmu_probe_armpmu(vcpu->cpu) != kvm_pmu_probe_armpmu(last_cpu))
+ kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
+}
+
void kvm_vcpu_reload_pmu(struct kvm_vcpu *vcpu)
{
u64 mask = kvm_pmu_implemented_counter_mask(vcpu);
@@ -1003,6 +1053,9 @@ u8 kvm_arm_pmu_get_max_counters(struct kvm *kvm)
{
struct arm_pmu *arm_pmu = kvm->arch.arm_pmu;
+ if (kvm_pmu_fixed_counters_only(kvm))
+ return 0;
+
/*
* PMUv3 requires that all event counters are capable of counting any
* event, though the same may not be true of non-PMUv3 hardware.
diff --git a/include/kvm/arm_pmu.h b/include/kvm/arm_pmu.h
index 0a36a3d5c894..9720fbd0eeb4 100644
--- a/include/kvm/arm_pmu.h
+++ b/include/kvm/arm_pmu.h
@@ -59,6 +59,7 @@ void kvm_pmu_software_increment(struct kvm_vcpu *vcpu, u64 val);
void kvm_pmu_handle_pmcr(struct kvm_vcpu *vcpu, u64 val);
void kvm_pmu_set_counter_event_type(struct kvm_vcpu *vcpu, u64 data,
u64 select_idx);
+void kvm_vcpu_load_pmu(struct kvm_vcpu *vcpu, int last_cpu);
void kvm_vcpu_reload_pmu(struct kvm_vcpu *vcpu);
int kvm_arm_pmu_v3_set_attr(struct kvm_vcpu *vcpu,
struct kvm_device_attr *attr);
@@ -164,6 +165,7 @@ static inline u64 kvm_pmu_get_pmceid(struct kvm_vcpu *vcpu, bool pmceid1)
static inline void kvm_pmu_update_vcpu_events(struct kvm_vcpu *vcpu) {}
static inline void kvm_vcpu_pmu_restore_guest(struct kvm_vcpu *vcpu) {}
static inline void kvm_vcpu_pmu_restore_host(struct kvm_vcpu *vcpu) {}
+static inline void kvm_vcpu_load_pmu(struct kvm_vcpu *vcpu, int last_cpu) {}
static inline void kvm_vcpu_reload_pmu(struct kvm_vcpu *vcpu) {}
static inline u8 kvm_arm_pmu_get_pmuver_limit(void)
{
--
2.55.0
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