KVM: arm64: PMU: Assume PMU presence in pmu-emul.c

Many functions in pmu-emul.c checks kvm_vcpu_has_pmu(vcpu). A favorable
interpretation is defensive programming, but it also has downsides:

- It is confusing as it implies these functions are called without PMU
  although most of them are called only when a PMU is present.

- It makes semantics of functions fuzzy. For example, calling
  kvm_pmu_disable_counter_mask() without PMU may result in no-op as
  there are no enabled counters, but it's unclear what
  kvm_pmu_get_counter_value() returns when there is no PMU.

- It allows callers without checking kvm_vcpu_has_pmu(vcpu), but it is
  often wrong to call these functions without PMU.

- It is error-prone to duplicate kvm_vcpu_has_pmu(vcpu) checks into
  multiple functions. Many functions are called for system registers,
  and the system register infrastructure already employs less
  error-prone, comprehensive checks.

Check kvm_vcpu_has_pmu(vcpu) in callers of these functions instead,
and remove the obsolete checks from pmu-emul.c. The only exceptions are
the functions that implement ioctls as they have definitive semantics
even when the PMU is not present.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250315-pmc-v5-2-ecee87dab216@daynix.com
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/arm.c

@@ -835,9 +835,11 @@ int kvm_arch_vcpu_run_pid_change(struct kvm_vcpu *vcpu)
 	if (ret)
 		return ret;
 
-	ret = kvm_arm_pmu_v3_enable(vcpu);
-	if (ret)
-		return ret;
+	if (kvm_vcpu_has_pmu(vcpu)) {
+		ret = kvm_arm_pmu_v3_enable(vcpu);
+		if (ret)
+			return ret;
+	}
 
 	if (is_protected_kvm_enabled()) {
 		ret = pkvm_create_hyp_vm(kvm);
@@ -1148,7 +1150,8 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
 		 */
 		preempt_disable();
 
-		kvm_pmu_flush_hwstate(vcpu);
+		if (kvm_vcpu_has_pmu(vcpu))
+			kvm_pmu_flush_hwstate(vcpu);
 
 		local_irq_disable();
 
@@ -1167,7 +1170,8 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
 		if (ret <= 0 || kvm_vcpu_exit_request(vcpu, &ret)) {
 			vcpu->mode = OUTSIDE_GUEST_MODE;
 			isb(); /* Ensure work in x_flush_hwstate is committed */
-			kvm_pmu_sync_hwstate(vcpu);
+			if (kvm_vcpu_has_pmu(vcpu))
+				kvm_pmu_sync_hwstate(vcpu);
 			if (unlikely(!irqchip_in_kernel(vcpu->kvm)))
 				kvm_timer_sync_user(vcpu);
 			kvm_vgic_sync_hwstate(vcpu);
@@ -1197,7 +1201,8 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
 		 * that the vgic can properly sample the updated state of the
 		 * interrupt line.
 		 */
-		kvm_pmu_sync_hwstate(vcpu);
+		if (kvm_vcpu_has_pmu(vcpu))
+			kvm_pmu_sync_hwstate(vcpu);
 
 		/*
 		 * Sync the vgic state before syncing the timer state because

arch/arm64/kvm/emulate-nested.c

@@ -2516,7 +2516,8 @@ void kvm_emulate_nested_eret(struct kvm_vcpu *vcpu)
 	kvm_arch_vcpu_load(vcpu, smp_processor_id());
 	preempt_enable();
 
-	kvm_pmu_nested_transition(vcpu);
+	if (kvm_vcpu_has_pmu(vcpu))
+		kvm_pmu_nested_transition(vcpu);
 }
 
 static void kvm_inject_el2_exception(struct kvm_vcpu *vcpu, u64 esr_el2,
@@ -2599,7 +2600,8 @@ static int kvm_inject_nested(struct kvm_vcpu *vcpu, u64 esr_el2,
 	kvm_arch_vcpu_load(vcpu, smp_processor_id());
 	preempt_enable();
 
-	kvm_pmu_nested_transition(vcpu);
+	if (kvm_vcpu_has_pmu(vcpu))
+		kvm_pmu_nested_transition(vcpu);
 
 	return 1;
 }

arch/arm64/kvm/pmu-emul.c

@@ -150,9 +150,6 @@ static u64 kvm_pmu_get_pmc_value(struct kvm_pmc *pmc)
  */
 u64 kvm_pmu_get_counter_value(struct kvm_vcpu *vcpu, u64 select_idx)
 {
-	if (!kvm_vcpu_has_pmu(vcpu))
-		return 0;
-
 	return kvm_pmu_get_pmc_value(kvm_vcpu_idx_to_pmc(vcpu, select_idx));
 }
 
@@ -191,9 +188,6 @@ static void kvm_pmu_set_pmc_value(struct kvm_pmc *pmc, u64 val, bool force)
  */
 void kvm_pmu_set_counter_value(struct kvm_vcpu *vcpu, u64 select_idx, u64 val)
 {
-	if (!kvm_vcpu_has_pmu(vcpu))
-		return;
-
 	kvm_pmu_set_pmc_value(kvm_vcpu_idx_to_pmc(vcpu, select_idx), val, false);
 }
 
@@ -350,7 +344,7 @@ void kvm_pmu_reprogram_counter_mask(struct kvm_vcpu *vcpu, u64 val)
 {
 	int i;
 
-	if (!kvm_vcpu_has_pmu(vcpu) || !val)
+	if (!val)
 		return;
 
 	for (i = 0; i < KVM_ARMV8_PMU_MAX_COUNTERS; i++) {
@@ -401,9 +395,6 @@ static void kvm_pmu_update_state(struct kvm_vcpu *vcpu)
 	struct kvm_pmu *pmu = &vcpu->arch.pmu;
 	bool overflow;
 
-	if (!kvm_vcpu_has_pmu(vcpu))
-		return;
-
 	overflow = kvm_pmu_overflow_status(vcpu);
 	if (pmu->irq_level == overflow)
 		return;
@@ -599,9 +590,6 @@ void kvm_pmu_handle_pmcr(struct kvm_vcpu *vcpu, u64 val)
 {
 	int i;
 
-	if (!kvm_vcpu_has_pmu(vcpu))
-		return;
-
 	/* Fixup PMCR_EL0 to reconcile the PMU version and the LP bit */
 	if (!kvm_has_feat(vcpu->kvm, ID_AA64DFR0_EL1, PMUVer, V3P5))
 		val &= ~ARMV8_PMU_PMCR_LP;
@@ -766,9 +754,6 @@ void kvm_pmu_set_counter_event_type(struct kvm_vcpu *vcpu, u64 data,
 	struct kvm_pmc *pmc = kvm_vcpu_idx_to_pmc(vcpu, select_idx);
 	u64 reg;
 
-	if (!kvm_vcpu_has_pmu(vcpu))
-		return;
-
 	reg = counter_index_to_evtreg(pmc->idx);
 	__vcpu_sys_reg(vcpu, reg) = data & kvm_pmu_evtyper_mask(vcpu->kvm);
 
@@ -848,9 +833,6 @@ u64 kvm_pmu_get_pmceid(struct kvm_vcpu *vcpu, bool pmceid1)
 	u64 val, mask = 0;
 	int base, i, nr_events;
 
-	if (!kvm_vcpu_has_pmu(vcpu))
-		return 0;
-
 	if (!pmceid1) {
 		val = read_sysreg(pmceid0_el0);
 		/* always support CHAIN */
@@ -900,9 +882,6 @@ void kvm_vcpu_reload_pmu(struct kvm_vcpu *vcpu)
 
 int kvm_arm_pmu_v3_enable(struct kvm_vcpu *vcpu)
 {
-	if (!kvm_vcpu_has_pmu(vcpu))
-		return 0;
-
 	if (!vcpu->arch.pmu.created)
 		return -EINVAL;
 
@@ -1231,9 +1210,6 @@ void kvm_pmu_nested_transition(struct kvm_vcpu *vcpu)
 	unsigned long mask;
 	int i;
 
-	if (!kvm_vcpu_has_pmu(vcpu))
-		return;
-
 	mask = __vcpu_sys_reg(vcpu, PMCNTENSET_EL0);
 	for_each_set_bit(i, &mask, 32) {
 		struct kvm_pmc *pmc = kvm_vcpu_idx_to_pmc(vcpu, i);

arch/arm64/kvm/sys_regs.c

@@ -1853,12 +1853,14 @@ static int set_id_aa64dfr0_el1(struct kvm_vcpu *vcpu,
 static u64 read_sanitised_id_dfr0_el1(struct kvm_vcpu *vcpu,
 				      const struct sys_reg_desc *rd)
 {
-	u8 perfmon = pmuver_to_perfmon(kvm_arm_pmu_get_pmuver_limit());
+	u8 perfmon;
 	u64 val = read_sanitised_ftr_reg(SYS_ID_DFR0_EL1);
 
 	val &= ~ID_DFR0_EL1_PerfMon_MASK;
-	if (kvm_vcpu_has_pmu(vcpu))
+	if (kvm_vcpu_has_pmu(vcpu)) {
+		perfmon = pmuver_to_perfmon(kvm_arm_pmu_get_pmuver_limit());
 		val |= SYS_FIELD_PREP(ID_DFR0_EL1, PerfMon, perfmon);
+	}
 
 	val = ID_REG_LIMIT_FIELD_ENUM(val, ID_DFR0_EL1, CopDbg, Debugv8p8);