[RFC PATCH 3/3] KVM: Expedite SRCU callbacks when freeing objects during I/O bus registration

Sean Christopherson
2026-03-09T19:31:18Z

Use the recently introduced call_srcu_expedited() when freeing the old I/O
bug during device registration to avoid triggering a non-expedited grace
period. Delaying the freeing of the object by a full grace period is a
complete non-issue, but the grace period also gets transferred to future
synchronizations, e.g. to the synchronize_srcu_expedited() invocation in
kvm_swap_active_memslots().

For micro-VM use cases, effectively transferring the non-expedited grace
period to memslot updates results in a meaningful delay in overall boot
time. E.g. with a CONFIG_HZ=100 kernel, the sync triggers a ~20ms delay,
increasing boot times by 15% or more.

Fixes: 7d9a0273c459 ("KVM: Avoid synchronize_srcu() in kvm_io_bus_register_dev()")
Reported-by: Nikita Kalyazin <kalyazin@amazon.com>
Closes: https://lore.kernel.org/all/a84ddba8-12da-489a-9dd1-ccdf7451a1ba@amazon.com
Cc: Keir Fraser <keirf@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
virt/kvm/kvm_main.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 9faf70ccae7a..ceaf08a03428 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -6021,7 +6021,7 @@ 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);
- call_srcu(&kvm->srcu, &bus->rcu, __free_bus);
+ call_srcu_expedited(&kvm->srcu, &bus->rcu, __free_bus);

return 0;
}
--
2.53.0.473.g4a7958ca14-goog