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Signed-off-by: Keir Fraser --- arch/arm64/kvm/vgic/vgic-init.c | 5 ++--- include/kvm/arm_vgic.h | 1 - 2 files changed, 2 insertions(+), 4 deletions(-) diff --git a/arch/arm64/kvm/vgic/vgic-init.c b/arch/arm64/kvm/vgic/vgic-ini= t.c index 1e680ad6e863..3f207b5f80a5 100644 --- a/arch/arm64/kvm/vgic/vgic-init.c +++ b/arch/arm64/kvm/vgic/vgic-init.c @@ -554,7 +554,6 @@ int vgic_lazy_init(struct kvm *kvm) * Also map the virtual CPU interface into the VM. * v2 calls vgic_init() if not already done. * v3 and derivatives return an error if the VGIC is not initialized. - * vgic_ready() returns true if this function has succeeded. */ int kvm_vgic_map_resources(struct kvm *kvm) { @@ -563,12 +562,12 @@ int kvm_vgic_map_resources(struct kvm *kvm) gpa_t dist_base; int ret =3D 0; =20 - if (likely(vgic_ready(kvm))) + if (likely(dist->ready)) return 0; =20 mutex_lock(&kvm->slots_lock); mutex_lock(&kvm->arch.config_lock); - if (vgic_ready(kvm)) + if (dist->ready) goto out; =20 if (!irqchip_in_kernel(kvm)) diff --git a/include/kvm/arm_vgic.h b/include/kvm/arm_vgic.h index 404883c7af6e..e7ffaf4bf2e7 100644 --- a/include/kvm/arm_vgic.h +++ b/include/kvm/arm_vgic.h @@ -406,7 +406,6 @@ u64 vgic_v3_get_misr(struct kvm_vcpu *vcpu); =20 #define irqchip_in_kernel(k) (!!((k)->arch.vgic.in_kernel)) #define vgic_initialized(k) ((k)->arch.vgic.initialized) -#define vgic_ready(k) ((k)->arch.vgic.ready) #define vgic_valid_spi(k, i) (((i) >=3D VGIC_NR_PRIVATE_IRQS) && \ ((i) < (k)->arch.vgic.nr_spis + VGIC_NR_PRIVATE_IRQS)) =20 --=20 2.51.0.384.g4c02a37b29-goog From nobody Thu Oct 2 23:57:56 2025 Received: from mail-wr1-f74.google.com (mail-wr1-f74.google.com [209.85.221.74]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 91D5A3148A2 for ; Tue, 9 Sep 2025 10:00:17 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.221.74 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757412019; cv=none; b=d+dA5aMyM1iA3nrd6mFJ2uOAfshJ6gzxkk+Fqcoacke/dq6EDI+TIm3Lqjet4kv3hPkhg7ZQF4WZfJr2lPMQ/fwra67wP+kr/eTuIFVIYjJeO6r2kUNNnvbw4coQu24Z95UWfeEBb57jKqUqU+MIfz5QikDc9P+fYUaUrrw0d+8= ARC-Message-Signature: i=1; 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charset="utf-8" In preparation to remove synchronize_srcu() from MMIO registration, remove the distributor's dependency on this implicit barrier by direct acquire-release synchronization on the flag write and its lock-free check. Signed-off-by: Keir Fraser --- arch/arm64/kvm/vgic/vgic-init.c | 11 ++--------- 1 file changed, 2 insertions(+), 9 deletions(-) diff --git a/arch/arm64/kvm/vgic/vgic-init.c b/arch/arm64/kvm/vgic/vgic-ini= t.c index 3f207b5f80a5..ccccb5c04ac1 100644 --- a/arch/arm64/kvm/vgic/vgic-init.c +++ b/arch/arm64/kvm/vgic/vgic-init.c @@ -562,7 +562,7 @@ int kvm_vgic_map_resources(struct kvm *kvm) gpa_t dist_base; int ret =3D 0; =20 - if (likely(dist->ready)) + if (likely(smp_load_acquire(&dist->ready))) return 0; =20 mutex_lock(&kvm->slots_lock); @@ -593,14 +593,7 @@ int kvm_vgic_map_resources(struct kvm *kvm) goto out_slots; } =20 - /* - * kvm_io_bus_register_dev() guarantees all readers see the new MMIO - * registration before returning through synchronize_srcu(), which also - * implies a full memory barrier. As such, marking the distributor as - * 'ready' here is guaranteed to be ordered after all vCPUs having seen - * a completely configured distributor. - */ - dist->ready =3D true; + smp_store_release(&dist->ready, true); goto out_slots; out: mutex_unlock(&kvm->arch.config_lock); --=20 2.51.0.384.g4c02a37b29-goog From nobody Thu Oct 2 23:57:56 2025 Received: from mail-wr1-f74.google.com (mail-wr1-f74.google.com [209.85.221.74]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 64BC93148DE for ; Tue, 9 Sep 2025 10:00:18 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.221.74 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757412020; cv=none; b=GmGuYJELkU8eqiaI6JpTXqDOZLp638HxzcIEX+OeyKdHS4/xkweaCEbYIGrkII4GebWIo8bUxuzS5IjZVdwzBkiwzg3TvYOWqZxh0Kcdk7QtapQsFQvkCtA4/IYUeAEpXTFRx4BCb6SeBGXelxIJuFAUKjRG3/OF4b6a+C4m3XY= ARC-Message-Signature: i=1; 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charset="utf-8" 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 --- arch/x86/kvm/vmx/vmx.c | 7 +++++++ include/linux/kvm_host.h | 10 +++++++--- virt/kvm/kvm_main.c | 32 ++++++++++++++++++++++++++------ 3 files changed, 40 insertions(+), 9 deletions(-) diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index aa157fe5b7b3..0bdf9405969a 100644 --- a/arch/x86/kvm/vmx/vmx.c +++ b/arch/x86/kvm/vmx/vmx.c @@ -5785,6 +5785,13 @@ static int handle_invalid_guest_state(struct kvm_vcp= u *vcpu) if (kvm_test_request(KVM_REQ_EVENT, vcpu)) return 1; =20 + /* + * Ensure that any updates to kvm->buses[] observed by the + * previous instruction (emulated or otherwise) are also + * visible to the instruction KVM is about to emulate. + */ + smp_rmb(); + if (!kvm_emulate_instruction(vcpu, 0)) return 0; =20 diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index 15656b7fba6c..e7d6111cf254 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -966,11 +966,15 @@ static inline bool kvm_dirty_log_manual_protect_and_i= nit_set(struct kvm *kvm) return !!(kvm->manual_dirty_log_protect & KVM_DIRTY_LOG_INITIALLY_SET); } =20 +/* + * 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)); } =20 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 6c07dd423458..870ad8ea93a7 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -1103,6 +1103,14 @@ void __weak kvm_arch_create_vm_debugfs(struct kvm *k= vm) { } =20 +/* 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 *k= vm, + 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 =3D kvm_arch_alloc_vm(); @@ -1228,7 +1236,7 @@ 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 =3D 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 +1284,7 @@ static void kvm_destroy_vm(struct kvm *kvm) =20 kvm_free_irq_routing(kvm); for (i =3D 0; i < KVM_NR_BUSES; i++) { - struct kvm_io_bus *bus =3D kvm_get_bus(kvm, i); + struct kvm_io_bus *bus =3D kvm_get_bus_for_destruction(kvm, i); =20 if (bus) kvm_io_bus_destroy(bus); @@ -5843,6 +5851,18 @@ static int __kvm_io_bus_write(struct kvm_vcpu *vcpu,= struct kvm_io_bus *bus, return -EOPNOTSUPP; } =20 +static struct kvm_io_bus *kvm_get_bus_srcu(struct kvm *kvm, enum kvm_bus i= dx) +{ + /* + * 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 ad= dr, int len, const void *val) { @@ -5855,7 +5875,7 @@ int kvm_io_bus_write(struct kvm_vcpu *vcpu, enum kvm_= bus bus_idx, gpa_t addr, .len =3D len, }; =20 - bus =3D srcu_dereference(vcpu->kvm->buses[bus_idx], &vcpu->kvm->srcu); + bus =3D kvm_get_bus_srcu(vcpu->kvm, bus_idx); if (!bus) return -ENOMEM; r =3D __kvm_io_bus_write(vcpu, bus, &range, val); @@ -5874,7 +5894,7 @@ int kvm_io_bus_write_cookie(struct kvm_vcpu *vcpu, en= um kvm_bus bus_idx, .len =3D len, }; =20 - bus =3D srcu_dereference(vcpu->kvm->buses[bus_idx], &vcpu->kvm->srcu); + bus =3D kvm_get_bus_srcu(vcpu->kvm, bus_idx); if (!bus) return -ENOMEM; =20 @@ -5924,7 +5944,7 @@ int kvm_io_bus_read(struct kvm_vcpu *vcpu, enum kvm_b= us bus_idx, gpa_t addr, .len =3D len, }; =20 - bus =3D srcu_dereference(vcpu->kvm->buses[bus_idx], &vcpu->kvm->srcu); + bus =3D kvm_get_bus_srcu(vcpu->kvm, bus_idx); 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AJvYcCVfYOBLtZvw1D5osSYFGL1adesCf6JGKwq3vYXeCqxAEDqSi4rWUyLGgNouj6PrOfos/7FT6BFRrjdLmKo=@vger.kernel.org X-Gm-Message-State: AOJu0Yx+4e9upxdhBed0xfqigIN6gUk4WfayUhLI/xHDGQsUXrYqyvrw ljD/7s2x/xLjWT0U/kYoB+bMhp2gH5RbZLOVly0wD5zkd1rDhCyR+3zWxLNg2wosNAOl8jBDvT9 GDQ== X-Google-Smtp-Source: AGHT+IFFKS9LSCg4pqICkObMpdq7WwENyXMbo01qMECVjQhfTtItSGy3G3y/UQTpd1+00goRvADrbyzv2A== X-Received: from wmbei14.prod.google.com ([2002:a05:600c:3f0e:b0:45d:e45e:96aa]) (user=keirf job=prod-delivery.src-stubby-dispatcher) by 2002:a05:600c:548a:b0:458:bfb1:1fb6 with SMTP id 5b1f17b1804b1-45ddde6a471mr83472825e9.2.1757412017523; 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> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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[]; }; =20 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); =20 +static void __free_bus(struct rcu_head *rcu) +{ + struct kvm_io_bus *bus =3D 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 a= ddr, 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); =20 return 0; } --=20 2.51.0.384.g4c02a37b29-goog