KVM: Never start grow vCPU halt_poll_ns from value below halt_poll_ns_grow_start

grow_halt_poll_ns() have a strange behaviour in case
(vcpu->halt_poll_ns != 0) &&
(vcpu->halt_poll_ns < halt_poll_ns_grow_start).

In this case, vcpu->halt_poll_ns will be multiplied by grow factor
(halt_poll_ns_grow) which will require several grow iteration in order
to reach a value bigger than halt_poll_ns_grow_start.
This means that growing vcpu->halt_poll_ns from value of 0 is slower
than growing it from a positive value less than halt_poll_ns_grow_start.
Which is misleading and inaccurate.

Fix issue by changing grow_halt_poll_ns() to set vcpu->halt_poll_ns
to halt_poll_ns_grow_start in any case that
(vcpu->halt_poll_ns < halt_poll_ns_grow_start).
Regardless if vcpu->halt_poll_ns is 0.

use READ_ONCE to get a consistent number for all cases.
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Nir Weiner <nir.weiner@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/powerpc/kvm/book3s_hv.c

@@ -3634,10 +3634,9 @@ static void grow_halt_poll_ns(struct kvmppc_vcore *vc)
 	if (!halt_poll_ns_grow)
 		return;
 
-	if (vc->halt_poll_ns == 0)
-		vc->halt_poll_ns = halt_poll_ns_grow_start;
-	else
 	vc->halt_poll_ns *= halt_poll_ns_grow;
+	if (vc->halt_poll_ns < halt_poll_ns_grow_start)
+		vc->halt_poll_ns = halt_poll_ns_grow_start;
 }
 
 static void shrink_halt_poll_ns(struct kvmppc_vcore *vc)

virt/kvm/kvm_main.c

@@ -2189,17 +2189,17 @@ void kvm_sigset_deactivate(struct kvm_vcpu *vcpu)
 
 static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
 {
-	unsigned int old, val, grow;
+	unsigned int old, val, grow, grow_start;
 
 	old = val = vcpu->halt_poll_ns;
+	grow_start = READ_ONCE(halt_poll_ns_grow_start);
 	grow = READ_ONCE(halt_poll_ns_grow);
 	if (!grow)
 		goto out;
 
-	if (val == 0)
-		val = halt_poll_ns_grow_start;
-	else
 	val *= grow;
+	if (val < grow_start)
+		val = grow_start;
 
 	if (val > halt_poll_ns)
 		val = halt_poll_ns;