1 files changed, 42 insertions, 10 deletions

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 25d7872b29c1..fab4d3790578 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1198,8 +1198,6 @@ static struct kvm *kvm_create_vm(unsigned long type, const char *fdname)
 			goto out_err_no_arch_destroy_vm;
 	}
 
-	kvm->max_halt_poll_ns = halt_poll_ns;
-
 	r = kvm_arch_init_vm(kvm, type);
 	if (r)
 		goto out_err_no_arch_destroy_vm;
@@ -3377,9 +3375,6 @@ static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
 	if (val < grow_start)
 		val = grow_start;
 
-	if (val > vcpu->kvm->max_halt_poll_ns)
-		val = vcpu->kvm->max_halt_poll_ns;
-
 	vcpu->halt_poll_ns = val;
 out:
 	trace_kvm_halt_poll_ns_grow(vcpu->vcpu_id, val, old);
@@ -3483,6 +3478,24 @@ static inline void update_halt_poll_stats(struct kvm_vcpu *vcpu, ktime_t start,
 	}
 }
 
+static unsigned int kvm_vcpu_max_halt_poll_ns(struct kvm_vcpu *vcpu)
+{
+	struct kvm *kvm = vcpu->kvm;
+
+	if (kvm->override_halt_poll_ns) {
+		/*
+		 * Ensure kvm->max_halt_poll_ns is not read before
+		 * kvm->override_halt_poll_ns.
+		 *
+		 * Pairs with the smp_wmb() when enabling KVM_CAP_HALT_POLL.
+		 */
+		smp_rmb();
+		return READ_ONCE(kvm->max_halt_poll_ns);
+	}
+
+	return READ_ONCE(halt_poll_ns);
+}
+
 /*
  * Emulate a vCPU halt condition, e.g. HLT on x86, WFI on arm, etc...  If halt
  * polling is enabled, busy wait for a short time before blocking to avoid the
@@ -3491,12 +3504,18 @@ static inline void update_halt_poll_stats(struct kvm_vcpu *vcpu, ktime_t start,
  */
 void kvm_vcpu_halt(struct kvm_vcpu *vcpu)
 {
+	unsigned int max_halt_poll_ns = kvm_vcpu_max_halt_poll_ns(vcpu);
 	bool halt_poll_allowed = !kvm_arch_no_poll(vcpu);
-	bool do_halt_poll = halt_poll_allowed && vcpu->halt_poll_ns;
 	ktime_t start, cur, poll_end;
 	bool waited = false;
+	bool do_halt_poll;
 	u64 halt_ns;
 
+	if (vcpu->halt_poll_ns > max_halt_poll_ns)
+		vcpu->halt_poll_ns = max_halt_poll_ns;
+
+	do_halt_poll = halt_poll_allowed && vcpu->halt_poll_ns;
+
 	start = cur = poll_end = ktime_get();
 	if (do_halt_poll) {
 		ktime_t stop = ktime_add_ns(start, vcpu->halt_poll_ns);
@@ -3535,18 +3554,21 @@ out:
 		update_halt_poll_stats(vcpu, start, poll_end, !waited);
 
 	if (halt_poll_allowed) {
+		/* Recompute the max halt poll time in case it changed. */
+		max_halt_poll_ns = kvm_vcpu_max_halt_poll_ns(vcpu);
+
 		if (!vcpu_valid_wakeup(vcpu)) {
 			shrink_halt_poll_ns(vcpu);
-		} else if (vcpu->kvm->max_halt_poll_ns) {
+		} else if (max_halt_poll_ns) {
 			if (halt_ns <= vcpu->halt_poll_ns)
 				;
 			/* we had a long block, shrink polling */
 			else if (vcpu->halt_poll_ns &&
-				 halt_ns > vcpu->kvm->max_halt_poll_ns)
+				 halt_ns > max_halt_poll_ns)
 				shrink_halt_poll_ns(vcpu);
 			/* we had a short halt and our poll time is too small */
-			else if (vcpu->halt_poll_ns < vcpu->kvm->max_halt_poll_ns &&
-				 halt_ns < vcpu->kvm->max_halt_poll_ns)
+			else if (vcpu->halt_poll_ns < max_halt_poll_ns &&
+				 halt_ns < max_halt_poll_ns)
 				grow_halt_poll_ns(vcpu);
 		} else {
 			vcpu->halt_poll_ns = 0;
@@ -4581,6 +4603,16 @@ static int kvm_vm_ioctl_enable_cap_generic(struct kvm *kvm,
 			return -EINVAL;
 
 		kvm->max_halt_poll_ns = cap->args[0];
+
+		/*
+		 * Ensure kvm->override_halt_poll_ns does not become visible
+		 * before kvm->max_halt_poll_ns.
+		 *
+		 * Pairs with the smp_rmb() in kvm_vcpu_max_halt_poll_ns().
+		 */
+		smp_wmb();
+		kvm->override_halt_poll_ns = true;
+
 		return 0;
 	}
 	case KVM_CAP_DIRTY_LOG_RING: