linux-kernel - [PATCH v2 4/4] KVM: Avoid synchronize_srcu() in kvm_io_bus_register

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Message-ID: <20250716110737.2513665-5-keirf@google.com>
Date: Wed, 16 Jul 2025 11:07:37 +0000
From: Keir Fraser <keirf@...gle.com>
To: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	kvm@...r.kernel.org
Cc: 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>, Keir Fraser <keirf@...gle.com>
Subject: [PATCH v2 4/4] KVM: Avoid synchronize_srcu() in kvm_io_bus_register_dev()

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 <keirf@...gle.com>
---
 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 9132148fb467..802ca46f7537 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 9ec3b96b9666..f690a4997a0f 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -5948,6 +5948,13 @@ 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)
 {
@@ -5986,8 +5993,7 @@ 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;
 }
-- 
2.50.0.727.gbf7dc18ff4-goog