Merge branch kvm-arm64/mmio-rcu into kvmarm-master/next

* kvm-arm64/mmio-rcu:
  : .
  : Speed up MMIO registration by avoiding unnecessary RCU synchronisation,
  : courtesy of Keir Fraser (20250909100007.3136249-1-keirf@google.com).
  : .
  KVM: Avoid synchronize_srcu() in kvm_io_bus_register_dev()
  KVM: Implement barriers before accessing kvm->buses[] on SRCU read paths
  KVM: arm64: vgic: Explicitly implement vgic_dist::ready ordering
  KVM: arm64: vgic-init: Remove vgic_ready() macro

Signed-off-by: Marc Zyngier <maz@kernel.org>

5 files changed, 53 insertions, 23 deletions

diff --git a/arch/arm64/kvm/vgic/vgic-init.c b/arch/arm64/kvm/vgic/vgic-init.c
index 1e680ad6e863..ccccb5c04ac1 100644
--- a/arch/arm64/kvm/vgic/vgic-init.c
+++ b/arch/arm64/kvm/vgic/vgic-init.c
@@ -554,7 +554,6 @@ int vgic_lazy_init(struct kvm *kvm)
  * Also map the virtual CPU interface into the VM.
  * v2 calls vgic_init() if not already done.
  * v3 and derivatives return an error if the VGIC is not initialized.
- * vgic_ready() returns true if this function has succeeded.
  */
 int kvm_vgic_map_resources(struct kvm *kvm)
 {
@@ -563,12 +562,12 @@ int kvm_vgic_map_resources(struct kvm *kvm)
 	gpa_t dist_base;
 	int ret = 0;
 
-	if (likely(vgic_ready(kvm)))
+	if (likely(smp_load_acquire(&dist->ready)))
 		return 0;
 
 	mutex_lock(&kvm->slots_lock);
 	mutex_lock(&kvm->arch.config_lock);
-	if (vgic_ready(kvm))
+	if (dist->ready)
 		goto out;
 
 	if (!irqchip_in_kernel(kvm))
@@ -594,14 +593,7 @@ int kvm_vgic_map_resources(struct kvm *kvm)
 		goto out_slots;
 	}
 
-	/*
-	 * kvm_io_bus_register_dev() guarantees all readers see the new MMIO
-	 * registration before returning through synchronize_srcu(), which also
-	 * implies a full memory barrier. As such, marking the distributor as
-	 * 'ready' here is guaranteed to be ordered after all vCPUs having seen
-	 * a completely configured distributor.
-	 */
-	dist->ready = true;
+	smp_store_release(&dist->ready, true);
 	goto out_slots;
 out:
 	mutex_unlock(&kvm->arch.config_lock);
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index aa157fe5b7b3..0bdf9405969a 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -5785,6 +5785,13 @@ static int handle_invalid_guest_state(struct kvm_vcpu *vcpu)
 		if (kvm_test_request(KVM_REQ_EVENT, vcpu))
 			return 1;
 
+		/*
+		 * Ensure that any updates to kvm->buses[] observed by the
+		 * previous instruction (emulated or otherwise) are also
+		 * visible to the instruction KVM is about to emulate.
+		 */
+		smp_rmb();
+
 		if (!kvm_emulate_instruction(vcpu, 0))
 			return 0;
 
diff --git a/include/kvm/arm_vgic.h b/include/kvm/arm_vgic.h
index 404883c7af6e..e7ffaf4bf2e7 100644
--- a/include/kvm/arm_vgic.h
+++ b/include/kvm/arm_vgic.h
@@ -406,7 +406,6 @@ u64 vgic_v3_get_misr(struct kvm_vcpu *vcpu);
 
 #define irqchip_in_kernel(k)	(!!((k)->arch.vgic.in_kernel))
 #define vgic_initialized(k)	((k)->arch.vgic.initialized)
-#define vgic_ready(k)		((k)->arch.vgic.ready)
 #define vgic_valid_spi(k, i)	(((i) >= VGIC_NR_PRIVATE_IRQS) && \
 			((i) < (k)->arch.vgic.nr_spis + VGIC_NR_PRIVATE_IRQS))
 
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 15656b7fba6c..103be35caf0d 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -206,6 +206,7 @@ struct kvm_io_range {
 struct kvm_io_bus {
 	int dev_count;
 	int ioeventfd_count;
+	struct rcu_head rcu;
 	struct kvm_io_range range[];
 };
 
@@ -966,11 +967,15 @@ static inline bool kvm_dirty_log_manual_protect_and_init_set(struct kvm *kvm)
 	return !!(kvm->manual_dirty_log_protect & KVM_DIRTY_LOG_INITIALLY_SET);
 }
 
+/*
+ * Get a bus reference under the update-side lock. No long-term SRCU reader
+ * references are permitted, to avoid stale reads vs concurrent IO
+ * registrations.
+ */
 static inline struct kvm_io_bus *kvm_get_bus(struct kvm *kvm, enum kvm_bus idx)
 {
-	return srcu_dereference_check(kvm->buses[idx], &kvm->srcu,
-				      lockdep_is_held(&kvm->slots_lock) ||
-				      !refcount_read(&kvm->users_count));
+	return rcu_dereference_protected(kvm->buses[idx],
+					 lockdep_is_held(&kvm->slots_lock));
 }
 
 static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 6c07dd423458..bcef324ccbf2 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1103,6 +1103,14 @@ void __weak kvm_arch_create_vm_debugfs(struct kvm *kvm)
 {
 }
 
+/* Called only on cleanup and destruction paths when there are no users. */
+static inline struct kvm_io_bus *kvm_get_bus_for_destruction(struct kvm *kvm,
+							     enum kvm_bus idx)
+{
+	return rcu_dereference_protected(kvm->buses[idx],
+					 !refcount_read(&kvm->users_count));
+}
+
 static struct kvm *kvm_create_vm(unsigned long type, const char *fdname)
 {
 	struct kvm *kvm = kvm_arch_alloc_vm();
@@ -1228,7 +1236,7 @@ out_err_no_disable:
 out_err_no_arch_destroy_vm:
 	WARN_ON_ONCE(!refcount_dec_and_test(&kvm->users_count));
 	for (i = 0; i < KVM_NR_BUSES; i++)
-		kfree(kvm_get_bus(kvm, i));
+		kfree(kvm_get_bus_for_destruction(kvm, i));
 	kvm_free_irq_routing(kvm);
 out_err_no_irq_routing:
 	cleanup_srcu_struct(&kvm->irq_srcu);
@@ -1276,7 +1284,7 @@ static void kvm_destroy_vm(struct kvm *kvm)
 
 	kvm_free_irq_routing(kvm);
 	for (i = 0; i < KVM_NR_BUSES; i++) {
-		struct kvm_io_bus *bus = kvm_get_bus(kvm, i);
+		struct kvm_io_bus *bus = kvm_get_bus_for_destruction(kvm, i);
 
 		if (bus)
 			kvm_io_bus_destroy(bus);
@@ -1312,6 +1320,7 @@ static void kvm_destroy_vm(struct kvm *kvm)
 		kvm_free_memslots(kvm, &kvm->__memslots[i][1]);
 	}
 	cleanup_srcu_struct(&kvm->irq_srcu);
+	srcu_barrier(&kvm->srcu);
 	cleanup_srcu_struct(&kvm->srcu);
 #ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
 	xa_destroy(&kvm->mem_attr_array);
@@ -5843,6 +5852,18 @@ static int __kvm_io_bus_write(struct kvm_vcpu *vcpu, struct kvm_io_bus *bus,
 	return -EOPNOTSUPP;
 }
 
+static struct kvm_io_bus *kvm_get_bus_srcu(struct kvm *kvm, enum kvm_bus idx)
+{
+	/*
+	 * Ensure that any updates to kvm_buses[] observed by the previous vCPU
+	 * machine instruction are also visible to the vCPU machine instruction
+	 * that triggered this call.
+	 */
+	smp_mb__after_srcu_read_lock();
+
+	return srcu_dereference(kvm->buses[idx], &kvm->srcu);
+}
+
 int kvm_io_bus_write(struct kvm_vcpu *vcpu, enum kvm_bus bus_idx, gpa_t addr,
 		     int len, const void *val)
 {
@@ -5855,7 +5876,7 @@ int kvm_io_bus_write(struct kvm_vcpu *vcpu, enum kvm_bus bus_idx, gpa_t addr,
 		.len = len,
 	};
 
-	bus = srcu_dereference(vcpu->kvm->buses[bus_idx], &vcpu->kvm->srcu);
+	bus = kvm_get_bus_srcu(vcpu->kvm, bus_idx);
 	if (!bus)
 		return -ENOMEM;
 	r = __kvm_io_bus_write(vcpu, bus, &range, val);
@@ -5874,7 +5895,7 @@ int kvm_io_bus_write_cookie(struct kvm_vcpu *vcpu, enum kvm_bus bus_idx,
 		.len = len,
 	};
 
-	bus = srcu_dereference(vcpu->kvm->buses[bus_idx], &vcpu->kvm->srcu);
+	bus = kvm_get_bus_srcu(vcpu->kvm, bus_idx);
 	if (!bus)
 		return -ENOMEM;
 
@@ -5924,7 +5945,7 @@ int kvm_io_bus_read(struct kvm_vcpu *vcpu, enum kvm_bus bus_idx, gpa_t addr,
 		.len = len,
 	};
 
-	bus = srcu_dereference(vcpu->kvm->buses[bus_idx], &vcpu->kvm->srcu);
+	bus = kvm_get_bus_srcu(vcpu->kvm, bus_idx);
 	if (!bus)
 		return -ENOMEM;
 	r = __kvm_io_bus_read(vcpu, bus, &range, val);
@@ -5932,6 +5953,13 @@ int kvm_io_bus_read(struct kvm_vcpu *vcpu, enum kvm_bus bus_idx, gpa_t addr,
 }
 EXPORT_SYMBOL_GPL(kvm_io_bus_read);
 
+static void __free_bus(struct rcu_head *rcu)
+{
+	struct kvm_io_bus *bus = container_of(rcu, struct kvm_io_bus, rcu);
+
+	kfree(bus);
+}
+
 int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
 			    int len, struct kvm_io_device *dev)
 {
@@ -5970,8 +5998,7 @@ int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
 	memcpy(new_bus->range + i + 1, bus->range + i,
 		(bus->dev_count - i) * sizeof(struct kvm_io_range));
 	rcu_assign_pointer(kvm->buses[bus_idx], new_bus);
-	synchronize_srcu_expedited(&kvm->srcu);
-	kfree(bus);
+	call_srcu(&kvm->srcu, &bus->rcu, __free_bus);
 
 	return 0;
 }
@@ -6033,7 +6060,7 @@ struct kvm_io_device *kvm_io_bus_get_dev(struct kvm *kvm, enum kvm_bus bus_idx,
 
 	srcu_idx = srcu_read_lock(&kvm->srcu);
 
-	bus = srcu_dereference(kvm->buses[bus_idx], &kvm->srcu);
+	bus = kvm_get_bus_srcu(kvm, bus_idx);
 	if (!bus)
 		goto out_unlock;