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@xxxxxxxxx>
Signed-off-by: Keir Fraser <keirf@xxxxxxxxxx>
---
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
