KVM: arm64: PMU: Simplify setting a counter to a specific value

kvm_pmu_set_counter_value() is pretty odd, as it tries to update
the counter value while taking into account the value that is
currently held by the running perf counter.

This is not only complicated, this is quite wrong. Nowhere in
the architecture is it said that the counter would be offset
by something that is pending. The counter should be updated
with the value set by SW, and start counting from there if
required.

Remove the odd computation and just assign the provided value
after having released the perf event (which is then restarted).
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221113163832.3154370-9-maz@kernel.org

arch/arm64/kvm/pmu-emul.c

@@ -23,6 +23,7 @@ static LIST_HEAD(arm_pmus);
 static DEFINE_MUTEX(arm_pmus_lock);
 
 static void kvm_pmu_create_perf_event(struct kvm_vcpu *vcpu, u64 select_idx);
+static void kvm_pmu_release_perf_event(struct kvm_pmc *pmc);
 
 static u32 kvm_pmu_event_mask(struct kvm *kvm)
 {
@@ -131,8 +132,10 @@ void kvm_pmu_set_counter_value(struct kvm_vcpu *vcpu, u64 select_idx, u64 val)
 	if (!kvm_vcpu_has_pmu(vcpu))
 		return;
 
+	kvm_pmu_release_perf_event(&vcpu->arch.pmu.pmc[select_idx]);
+
 	reg = counter_index_to_reg(select_idx);
-	__vcpu_sys_reg(vcpu, reg) += (s64)val - kvm_pmu_get_counter_value(vcpu, select_idx);
+	__vcpu_sys_reg(vcpu, reg) = val;
 
 	/* Recreate the perf event to reflect the updated sample_period */
 	kvm_pmu_create_perf_event(vcpu, select_idx);