KVM: grow_halt_poll_ns() should never shrink vCPU halt_poll_ns

grow_halt_poll_ns() have a strange behavior in case
(halt_poll_ns_grow == 0) && (vcpu->halt_poll_ns != 0).

In this case, vcpu->halt_pol_ns will be set to zero.
That results in shrinking instead of growing.

Fix issue by changing grow_halt_poll_ns() to not modify
vcpu->halt_poll_ns in case halt_poll_ns_grow is zero
Reviewed-by: NBoris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: NLiran Alon <liran.alon@oracle.com>
Signed-off-by: NNir Weiner <nir.weiner@oracle.com>
Suggested-by: NLiran Alon <liran.alon@oracle.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/powerpc/kvm/book3s_hv.c

@@ -3631,8 +3631,11 @@ static void kvmppc_wait_for_exec(struct kvmppc_vcore *vc,
 
 static void grow_halt_poll_ns(struct kvmppc_vcore *vc)
 {
+	if (!halt_poll_ns_grow)
+		return;
+
 	/* 10us base */
-	if (vc->halt_poll_ns == 0 && halt_poll_ns_grow)
+	if (vc->halt_poll_ns == 0)
 		vc->halt_poll_ns = 10000;
 	else
 		vc->halt_poll_ns *= halt_poll_ns_grow;

virt/kvm/kvm_main.c

@@ -2188,8 +2188,11 @@ static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
 
 	old = val = vcpu->halt_poll_ns;
 	grow = READ_ONCE(halt_poll_ns_grow);
+	if (!grow)
+		goto out;
+
 	/* 10us base */
-	if (val == 0 && grow)
+	if (val == 0)
 		val = 10000;
 	else
 		val *= grow;
@@ -2198,6 +2201,7 @@ static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
 		val = halt_poll_ns;
 
 	vcpu->halt_poll_ns = val;
+out:
 	trace_kvm_halt_poll_ns_grow(vcpu->vcpu_id, val, old);
 }