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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.

Tested-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Keir Fraser <keirf@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
This commit is contained in:
Keir Fraser 2025-09-09 10:00:07 +00:00 committed by Marc Zyngier
parent 7788255aba
commit 7d9a0273c4
2 changed files with 10 additions and 2 deletions
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1
include/linux/kvm_host.h
View File

@ -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[];
};

11
virt/kvm/kvm_main.c
View File

@ -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;
}