KVM: arm64: PMU: Do not let AArch32 change the counters' top 32 bits

Even when using PMUv3p5 (which implies 64bit counters), there is
no way for AArch32 to write to the top 32 bits of the counters.
The only way to influence these bits (other than by counting
events) is by writing PMCR.P==1.

Make sure we obey the architecture and preserve the top 32 bits
on a counter update.
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221113163832.3154370-10-maz@kernel.org

arch/arm64/kvm/pmu-emul.c

@@ -119,13 +119,8 @@ u64 kvm_pmu_get_counter_value(struct kvm_vcpu *vcpu, u64 select_idx)
 	return counter;
 }
 
-/**
- * kvm_pmu_set_counter_value - set PMU counter value
- * @vcpu: The vcpu pointer
- * @select_idx: The counter index
- * @val: The counter value
- */
-void kvm_pmu_set_counter_value(struct kvm_vcpu *vcpu, u64 select_idx, u64 val)
+static void kvm_pmu_set_counter(struct kvm_vcpu *vcpu, u64 select_idx, u64 val,
+				bool force)
 {
 	u64 reg;
 
@@ -135,12 +130,36 @@ void kvm_pmu_set_counter_value(struct kvm_vcpu *vcpu, u64 select_idx, u64 val)
 	kvm_pmu_release_perf_event(&vcpu->arch.pmu.pmc[select_idx]);
 
 	reg = counter_index_to_reg(select_idx);
+
+	if (vcpu_mode_is_32bit(vcpu) && select_idx != ARMV8_PMU_CYCLE_IDX &&
+	    !force) {
+		/*
+		 * Even with PMUv3p5, AArch32 cannot write to the top
+		 * 32bit of the counters. The only possible course of
+		 * action is to use PMCR.P, which will reset them to
+		 * 0 (the only use of the 'force' parameter).
+		 */
+		val  = __vcpu_sys_reg(vcpu, reg) & GENMASK(63, 32);
+		val |= lower_32_bits(val);
+	}
+
 	__vcpu_sys_reg(vcpu, reg) = val;
 
 	/* Recreate the perf event to reflect the updated sample_period */
 	kvm_pmu_create_perf_event(vcpu, select_idx);
 }
 
+/**
+ * kvm_pmu_set_counter_value - set PMU counter value
+ * @vcpu: The vcpu pointer
+ * @select_idx: The counter index
+ * @val: The counter value
+ */
+void kvm_pmu_set_counter_value(struct kvm_vcpu *vcpu, u64 select_idx, u64 val)
+{
+	kvm_pmu_set_counter(vcpu, select_idx, val, false);
+}
+
 /**
  * kvm_pmu_release_perf_event - remove the perf event
  * @pmc: The PMU counter pointer
@@ -533,7 +552,7 @@ void kvm_pmu_handle_pmcr(struct kvm_vcpu *vcpu, u64 val)
 		unsigned long mask = kvm_pmu_valid_counter_mask(vcpu);
 		mask &= ~BIT(ARMV8_PMU_CYCLE_IDX);
 		for_each_set_bit(i, &mask, 32)
-			kvm_pmu_set_counter_value(vcpu, i, 0);
+			kvm_pmu_set_counter(vcpu, i, 0, true);
 	}
 }