KVM: Refactor and document halt-polling stats update helper

Add a comment to document that halt-polling is considered successful even
if the polling loop itself didn't detect a wake event, i.e. if a wake
event was detect in the final kvm_vcpu_check_block().  Invert the param
to update helper so that the helper is a dumb function that is "told"
whether or not polling was successful, as opposed to determining success
based on blocking behavior.

Opportunistically tweak the params to the update helper to reduce the
line length for the call site so that it fits on a single line, and so
that the prototype conforms to the more traditional kernel style.

No functional change intended.
Reviewed-by: David Matlack <dmatlack@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211009021236.4122790-7-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -3272,13 +3272,15 @@ static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
 	return ret;
 }
 
-static inline void
-update_halt_poll_stats(struct kvm_vcpu *vcpu, u64 poll_ns, bool waited)
+static inline void update_halt_poll_stats(struct kvm_vcpu *vcpu, ktime_t start,
+					  ktime_t end, bool success)
 {
-	if (waited)
-		vcpu->stat.generic.halt_poll_fail_ns += poll_ns;
-	else
+	u64 poll_ns = ktime_to_ns(ktime_sub(end, start));
+
+	if (success)
 		vcpu->stat.generic.halt_poll_success_ns += poll_ns;
+	else
+		vcpu->stat.generic.halt_poll_fail_ns += poll_ns;
 }
 
 /*
@@ -3348,9 +3350,13 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
 	kvm_arch_vcpu_unblocking(vcpu);
 	block_ns = ktime_to_ns(cur) - ktime_to_ns(start);
 
+	/*
+	 * Note, halt-polling is considered successful so long as the vCPU was
+	 * never actually scheduled out, i.e. even if the wake event arrived
+	 * after of the halt-polling loop itself, but before the full wait.
+	 */
 	if (do_halt_poll)
-		update_halt_poll_stats(
-			vcpu, ktime_to_ns(ktime_sub(poll_end, start)), waited);
+		update_halt_poll_stats(vcpu, start, poll_end, !waited);
 
 	if (halt_poll_allowed) {
 		if (!vcpu_valid_wakeup(vcpu)) {