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Message-ID: <knb7uh5fk2okmszyncigplu2drimnfaprnjiburfsip2fnkce2@c2czybicw22d>
Date: Sat, 19 Jul 2025 10:33:19 +0800
From: Yao Yuan <yaoyuan0329os@...il.com>
To: Keir Fraser <keirf@...gle.com>
Cc: Yao Yuan <yaoyuan@...ux.alibaba.com>,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
Sean Christopherson <seanjc@...gle.com>, Eric Auger <eric.auger@...hat.com>,
Oliver Upton <oliver.upton@...ux.dev>, Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v2 3/4] KVM: Implement barriers before accessing
kvm->buses[] on SRCU read paths
On Fri, Jul 18, 2025 at 02:56:25PM +0000, Keir Fraser wrote:
> 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@...gle.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).
I see, thanks for your explanation!
>
> -- 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
> > >
>
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