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Tue, 09 Sep 2025 03:00:17 -0700 (PDT) Date: Tue, 9 Sep 2025 10:00:07 +0000 In-Reply-To: <20250909100007.3136249-1-keirf@google.com> Mime-Version: 1.0 References: <20250909100007.3136249-1-keirf@google.com> X-Mailer: git-send-email 2.51.0.384.g4c02a37b29-goog Message-ID: <20250909100007.3136249-5-keirf@google.com> Subject: [PATCH v4 4/4] KVM: Avoid synchronize_srcu() in kvm_io_bus_register_dev() From: Keir Fraser To: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Cc: Sean Christopherson , Eric Auger , Oliver Upton , Marc Zyngier , Will Deacon , Paolo Bonzini , Keir Fraser , Li RongQing Content-Type: text/plain; charset="UTF-8" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250909_110019_888762_A2CC58F7 X-CRM114-Status: GOOD ( 13.29 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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. Tested-by: Li RongQing Signed-off-by: Keir Fraser --- include/linux/kvm_host.h | 1 + virt/kvm/kvm_main.c | 11 +++++++++-- 2 files changed, 10 insertions(+), 2 deletions(-) diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index e7d6111cf254..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[]; }; diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 870ad8ea93a7..bcef324ccbf2 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -1320,6 +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); @@ -5952,6 +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) { @@ -5990,8 +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; } -- 2.51.0.384.g4c02a37b29-goog