KVM: Obey kvm.halt_poll_ns in VMs not using KVM_CAP_HALT_POLL

Obey kvm.halt_poll_ns in VMs not using KVM_CAP_HALT_POLL on every halt, rather than just sampling the module parameter when the VM is first created. This restore the original behavior of kvm.halt_poll_ns for VMs that have not opted into KVM_CAP_HALT_POLL. Notably, this change restores the ability for admins to disable or change the maximum halt-polling time system wide for VMs not using KVM_CAP_HALT_POLL. Reported-by: Christian Borntraeger <borntraeger@de.ibm.com> Fixes: acd0578 ("kvm: add capability for halt polling") Signed-off-by: David Matlack <dmatlack@google.com> Message-Id: <20221117001657.1067231-4-dmatlack@google.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Gnurou · Nov 17, 2022 · 9eb8ca0 · 9eb8ca0
1 parent 175d5dc
commit 9eb8ca0
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Showing 2 changed files with 25 additions and 3 deletions.
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
@@ -776,6 +776,7 @@ struct kvm {
 	struct srcu_struct srcu;
 	struct srcu_struct irq_srcu;
 	pid_t userspace_pid;
+	bool override_halt_poll_ns;
 	unsigned int max_halt_poll_ns;
 	u32 dirty_ring_size;
 	bool vm_bugged;

diff --git 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;
@@ -3482,7 +3480,20 @@ 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)
 {
-	return READ_ONCE(vcpu->kvm->max_halt_poll_ns);
+	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);
 }
 
 /*
@@ -4592,6 +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: