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[35.205.142.120]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-454dd439326sm9964525e9.1.2025.07.09.23.51.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 09 Jul 2025 23:51:20 -0700 (PDT) Date: Thu, 10 Jul 2025 06:51:16 +0000 From: Keir Fraser To: Sean Christopherson Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Eric Auger , Oliver Upton , Marc Zyngier , Will Deacon , Paolo Bonzini , Li RongQing Subject: Re: [PATCH 3/3] KVM: Avoid synchronize_srcu() in kvm_io_bus_register_dev() Message-ID: References: <20250624092256.1105524-1-keirf@google.com> <20250624092256.1105524-4-keirf@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Mon, Jul 07, 2025 at 11:49:34AM -0700, Sean Christopherson wrote: > On Mon, Jun 30, 2025, Keir Fraser wrote: > > On Tue, Jun 24, 2025 at 08:11:49AM -0700, Sean Christopherson wrote: > > > +Li > > > > > > On Tue, Jun 24, 2025, Keir Fraser wrote: > > > > Device MMIO registration may happen quite frequently during VM boot, > > > > and the SRCU synchronization each time has a measurable effect > > > > on VM startup time. In our experiments it can account for around 25% > > > > of a VM's startup time. > > > > > > > > Replace the synchronization with a deferred free of the old kvm_io_bus > > > > structure. > > > > > > > > Signed-off-by: Keir Fraser > > > > --- > > > > include/linux/kvm_host.h | 1 + > > > > virt/kvm/kvm_main.c | 10 ++++++++-- > > > > 2 files changed, 9 insertions(+), 2 deletions(-) > > > > > > > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > > > > index 3bde4fb5c6aa..28a63f1ad314 100644 > > > > --- a/include/linux/kvm_host.h > > > > +++ b/include/linux/kvm_host.h > > > > @@ -205,6 +205,7 @@ struct kvm_io_range { > > > > struct kvm_io_bus { > > > > int dev_count; > > > > int ioeventfd_count; > > > > + struct rcu_head rcu; > > > > struct kvm_io_range range[]; > > > > }; > > > > > > > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > > > > index eec82775c5bf..b7d4da8ba0b2 100644 > > > > --- a/virt/kvm/kvm_main.c > > > > +++ b/virt/kvm/kvm_main.c > > > > @@ -5924,6 +5924,13 @@ 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) > > > > { > > > > @@ -5962,8 +5969,7 @@ 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); > > > > > > I'm 99% certain this will break ABI. KVM needs to ensure all readers are > > > guaranteed to see the new device prior to returning to userspace. > > > > I'm not sure I understand this. How can userspace (or a guest VCPU) know that > > it is executing *after* the MMIO registration, except via some form of > > synchronization or other ordering of its own? For example, that PCI BAR setup > > happens as part of PCI probing happening early in device registration in the > > guest OS, strictly before the MMIO region will be accessed. Otherwise the > > access is inherently racy against the registration? > > Yes, guest software needs its own synchronization. What I am pointing out is that, > very strictly speaking, KVM relies on synchronize_srcu_expedited() to ensure that > KVM's emulation of MMIO accesses are correctly ordered with respect to the guest's > synchronization. > > It's legal, though *extremely* uncommon, for KVM to emulate large swaths of guest > code, including emulated MMIO accesses. If KVM grabs kvm->buses at the start of > an emulation block, and then uses that reference to resolve MMIO, it's theoretically > possible for KVM to mishandle an access due to using a stale bus. But it doesn't do that? I think I understand now though that you are concerned about the buses API exposed to the kernel at large. And yes I see this would be a problem for example if a kvm_get_bus() return value was cached. Will and I also had a brainstorm in the office and theorised that a really "smart" compiler might somehow unroll handle_invalid_guest_state() 130 times and hoist all the READ_ONCE()s of the bus to the start. It's impractical, even likely impossible, but we couldn't outright say it's disallowed by the current enforcements in the KVM subsystem itself. > Today, such scenarios are effectively prevented by synchronize_srcu_expedited(). > Using kvm->buses outside of SRCU protection would be a bug (per KVM's locking > rules), i.e. a large emulation block must take and hold SRCU for its entire > duration. And so waiting for all SRCU readers to go away ensures that the new > kvm->buses will be observed if KVM starts a new emulation block. Understood. Yes that does make the current code definitely safe in this regard, just slow! > AFAIK, the only example of such emulation is x86's handle_invalid_guest_state(). > And in practice, it's probably impossible for the compiler to keep a reference to > kvm->buses across multiple invocations of kvm_emulate_instruction() while still > honoring the READ_ONCE() in __rcu_dereference_check(). That certainly stops compiler reordering. But I think I now agree with your concern about needing an actual memory barrier. I am worried for example if the guest is relying on an address dependency to synchronise an MMIO access. For example: data = READ_ONCE(*READ_ONCE(mmio_base_addr)); Two loads, ordered by address dependency. But that dependency would not prevent the access to kvm->buses[idx] from being hoisted/speculated by the CPU, since it's not data-dependent on the preceding load... That said, on x86, loads are ordered anyway. > But I don't want to simply drop KVM's synchronization, because we need a rule of > some kind to ensure correct ordering, even if it's only for documentation purposes > for 99% of cases. And because the existence of kvm_get_bus() means that it would > be possible for KVM to grab a long-term reference to kvm->buses and use it across > emulation of multiple instructions (though actually doing that would be all kinds > of crazy). That seems reasonable, in terms of maintaining a fool-proof API to kvm->buses. > > > > I'm quite confident there are other flows that rely on the synchronization, > > > the vGIC case is simply the one that's documented. > > > > If they're in the kernel they can be fixed? If necessary I'll go audit the callers. > > Yes, I'm sure there's a solution. Thinking more about this, you make a good > point that KVM needs to order access with respect to instruction execution, not > with respect to the start of KVM_RUN. > > For all intents and purposes, holding kvm->srcu across VM-Enter/VM-Exit is > disallowed (though I don't think this is formally documented), i.e. every > architecture is guaranteed to do srcu_read_lock() after a VM-Exit, prior to > reading kvm->buses. And srcu_read_lock() contains a full smp_mb(), which ensures > KVM will get a fresh kvm->buses relative to the instruction that triggered the > exit. > > So for the common case of one-off accesses after a VM-Exit, I think we can simply > add calls to smp_mb__after_srcu_read_lock() (which is a nop on all architectures) > to formalize the dependency on reacquiring SRCU. AFAICT, that would also suffice > for arm64's use of kvm_io_bus_get_dev(). And then add an explicit barrier of some > kind in handle_invalid_guest_state()? > > Then to prevent grabbing long-term references to a bus, require kvm->slots_lock > in kvm_get_bus() (and special case the kfree() in VM destruction). > > So something like this? I think the barriers would pair with the smp_store_release() > in rcu_assign_pointer()? It would certainly mean that kvm->buses would be accessed *after* any preceding vCPU memory access. Including any that "observed" the newly-registered IO region, somehow (lock acquisition, read of a flag or base address, or whatever). Would it be satisfactory to put a patch along the lines of your suggestions below into a v2 of this patch series? I have made some comments below. > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c > index 4953846cb30d..057fb4ce66b0 100644 > --- a/arch/x86/kvm/vmx/vmx.c > +++ b/arch/x86/kvm/vmx/vmx.c > @@ -5861,6 +5861,9 @@ static int handle_invalid_guest_state(struct kvm_vcpu *vcpu) > if (kvm_test_request(KVM_REQ_EVENT, vcpu)) > return 1; > > + /* Or maybe smp_mb()? Not sure what this needs to be. */ > + barrier(); > + Looks weak but maybe strong enough for x86? Maybe smp_rmb() would be better statement of intention? > if (!kvm_emulate_instruction(vcpu, 0)) > return 0; > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > index 3bde4fb5c6aa..066438b6571a 100644 > --- a/include/linux/kvm_host.h > +++ b/include/linux/kvm_host.h > @@ -967,9 +967,8 @@ static inline bool kvm_dirty_log_manual_protect_and_init_set(struct kvm *kvm) > > 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 eec82775c5bf..7b0e881351f7 100644 > --- a/virt/kvm/kvm_main.c > +++ b/virt/kvm/kvm_main.c > @@ -1228,7 +1228,8 @@ 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(rcu_dereference_check(kvm->buses[i], &kvm->srcu, > + !refcount_read(&kvm->users_count)); srcu_dereference_check() > kvm_free_irq_routing(kvm); > out_err_no_irq_routing: > cleanup_srcu_struct(&kvm->irq_srcu); > @@ -5847,6 +5848,9 @@ int kvm_io_bus_write(struct kvm_vcpu *vcpu, enum kvm_bus bus_idx, gpa_t addr, > .len = len, > }; > > + /* comment goes here */ > + smp_mb__after_srcu_read_lock(); > + > bus = srcu_dereference(vcpu->kvm->buses[bus_idx], &vcpu->kvm->srcu); > if (!bus) > return -ENOMEM; > @@ -5866,6 +5870,9 @@ int kvm_io_bus_write_cookie(struct kvm_vcpu *vcpu, enum kvm_bus bus_idx, > .len = len, > }; > > + /* comment goes here */ > + smp_mb__after_srcu_read_lock(); > + > bus = srcu_dereference(vcpu->kvm->buses[bus_idx], &vcpu->kvm->srcu); > if (!bus) > return -ENOMEM; > @@ -6025,6 +6032,9 @@ struct kvm_io_device *kvm_io_bus_get_dev(struct kvm *kvm, enum kvm_bus bus_idx, > > srcu_idx = srcu_read_lock(&kvm->srcu); > > + /* comment goes here */ > + smp_mb__after_srcu_read_lock(); > + > bus = srcu_dereference(kvm->buses[bus_idx], &kvm->srcu); > if (!bus) > goto out_unlock; > > I guess kvm_io_bus_read() is to be done as well? Perhaps the barrier and dereference should be pulled into a helper with the comment, just in one place? -- Keir (with thanks to Will for brainstorming!)