[PATCH v2 3/4] KVM: Implement barriers before accessing kvm->buses[] on SRCU read paths

[Yao Yuan]

On Wed, Jul 16, 2025 at 11:07:36AM +0800, Keir Fraser wrote:
> This ensures that, if a VCPU has "observed" that an IO registration has
> occurred, the instruction currently being trapped or emulated will also
> observe the IO registration.
>
> At the same time, enforce that kvm_get_bus() is used only on the
> update side, ensuring that a long-term reference cannot be obtained by
> an SRCU reader.
>
> Signed-off-by: Keir Fraser <keirf at google.com>
> ---
>  arch/x86/kvm/vmx/vmx.c   |  7 +++++++
>  include/linux/kvm_host.h | 10 +++++++---
>  virt/kvm/kvm_main.c      | 33 +++++++++++++++++++++++++++------
>  3 files changed, 41 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 191a9ed0da22..425e3d8074ab 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -5861,6 +5861,13 @@ 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 we are about to emulate.
> +		 */
> +		smp_rmb();
> +
>  		if (!kvm_emulate_instruction(vcpu, 0))
>  			return 0;
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 3bde4fb5c6aa..9132148fb467 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -965,11 +965,15 @@ 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));

I want to consult the true reason for using protected version here,
save unnecessary READ_ONCE() ?

>  }
>
>  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 222f0e894a0c..9ec3b96b9666 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1103,6 +1103,15 @@ 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 +1237,7 @@ static struct kvm *kvm_create_vm(unsigned long type, const char *fdname)
>  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 +1285,7 @@ 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);
> @@ -5838,6 +5847,18 @@ 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)
>  {
> @@ -5850,7 +5871,7 @@ 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);
> @@ -5869,7 +5890,7 @@ 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;
>
> @@ -5919,7 +5940,7 @@ 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);
> @@ -6028,7 +6049,7 @@ 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;
>
> --
> 2.50.0.727.gbf7dc18ff4-goog
>