KVM: arm64: Compute PMCEID from arm_pmu's event bitmaps

The PMUv3 driver populates a couple of bitmaps with the values of
PMCEID{0,1}, from which the guest's PMCEID{0,1} can be derived. This
is particularly convenient when virtualizing PMUv3 on IMP DEF hardware,
as reading the nonexistent PMCEID registers leads to a rather unpleasant
UNDEF.

Tested-by: Janne Grunau <j@jannau.net>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250305202641.428114-4-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

Author: oupton

arch/arm64/kvm/pmu-emul.c

@@ -842,8 +842,42 @@ static struct arm_pmu *kvm_pmu_probe_armpmu(void)
 	return pmu;
 }
 
+static u64 __compute_pmceid(struct arm_pmu *pmu, bool pmceid1)
+{
+	u32 hi[2], lo[2];
+
+	bitmap_to_arr32(lo, pmu->pmceid_bitmap, ARMV8_PMUV3_MAX_COMMON_EVENTS);
+	bitmap_to_arr32(hi, pmu->pmceid_ext_bitmap, ARMV8_PMUV3_MAX_COMMON_EVENTS);
+
+	return ((u64)hi[pmceid1] << 32) | lo[pmceid1];
+}
+
+static u64 compute_pmceid0(struct arm_pmu *pmu)
+{
+	u64 val = __compute_pmceid(pmu, 0);
+
+	/* always support CHAIN */
+	val |= BIT(ARMV8_PMUV3_PERFCTR_CHAIN);
+	return val;
+}
+
+static u64 compute_pmceid1(struct arm_pmu *pmu)
+{
+	u64 val = __compute_pmceid(pmu, 1);
+
+	/*
+	 * Don't advertise STALL_SLOT*, as PMMIR_EL0 is handled
+	 * as RAZ
+	 */
+	val &= ~(BIT_ULL(ARMV8_PMUV3_PERFCTR_STALL_SLOT - 32) |
+		 BIT_ULL(ARMV8_PMUV3_PERFCTR_STALL_SLOT_FRONTEND - 32) |
+		 BIT_ULL(ARMV8_PMUV3_PERFCTR_STALL_SLOT_BACKEND - 32));
+	return val;
+}
+
 u64 kvm_pmu_get_pmceid(struct kvm_vcpu *vcpu, bool pmceid1)
 {
+	struct arm_pmu *cpu_pmu = vcpu->kvm->arch.arm_pmu;
 	unsigned long *bmap = vcpu->kvm->arch.pmu_filter;
 	u64 val, mask = 0;
 	int base, i, nr_events;
@@ -852,19 +886,10 @@ u64 kvm_pmu_get_pmceid(struct kvm_vcpu *vcpu, bool pmceid1)
 		return 0;
 
 	if (!pmceid1) {
-		val = read_sysreg(pmceid0_el0);
-		/* always support CHAIN */
-		val |= BIT(ARMV8_PMUV3_PERFCTR_CHAIN);
+		val = compute_pmceid0(cpu_pmu);
 		base = 0;
 	} else {
-		val = read_sysreg(pmceid1_el0);
-		/*
-		 * Don't advertise STALL_SLOT*, as PMMIR_EL0 is handled
-		 * as RAZ
-		 */
-		val &= ~(BIT_ULL(ARMV8_PMUV3_PERFCTR_STALL_SLOT - 32) |
-			 BIT_ULL(ARMV8_PMUV3_PERFCTR_STALL_SLOT_FRONTEND - 32) |
-			 BIT_ULL(ARMV8_PMUV3_PERFCTR_STALL_SLOT_BACKEND - 32));
+		val = compute_pmceid1(cpu_pmu);
 		base = 32;
 	}