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

[Sean Christopherson]

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@xxxxxxxxxx>
Closes: https://lore.kernel.org/all/a84ddba8-12da-489a-9dd1-ccdf7451a1ba@xxxxxxxxxx
Cc: Keir Fraser <keirf@xxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 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