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

[Keir Fraser <keirf@google.com>]

On Thu, Jul 17, 2025 at 02:01:32PM +0800, Yao Yuan wrote:
> 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@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() ?

We don't want this function to be callable from SRCU read section, but
*only* during teardown. Hence protected version provides a better,
stricter safety check (that there are no users).

 -- Keir

> >  }
> >
> >  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
> >