KVM: irqfd: Make resampler_list an RCU list

It is useful to be able to do read-only traversal of the list of all the
registered irqfd resamplers without locking the resampler_lock mutex.
In particular, we are going to traverse it to search for a resampler
registered for the given irq of an irqchip, and that will be done with
an irqchip spinlock (ioapic->lock) held, so it is undesirable to lock a
mutex in this context. So turn this list into an RCU list.

For protecting the read side, reuse kvm->irq_srcu which is already used
for protecting a number of irq related things (kvm->irq_routing,
irqfd->resampler->list, kvm->irq_ack_notifier_list,
kvm->arch.mask_notifier_list).

Signed-off-by: Dmytro Maluka <dmy@semihalf.com>
Message-Id: <20230322204344.50138-2-dmy@semihalf.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

2 files changed, 2 insertions, 1 deletions

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 8ada23756b0e..9f508c8e66e1 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -755,6 +755,7 @@ struct kvm {
 	struct {
 		spinlock_t        lock;
 		struct list_head  items;
+		/* resampler_list update side is protected by resampler_lock. */
 		struct list_head  resampler_list;
 		struct mutex      resampler_lock;
 	} irqfds;
diff --git a/include/linux/kvm_irqfd.h b/include/linux/kvm_irqfd.h
index dac047abdba7..8ad43692e3bb 100644
--- a/include/linux/kvm_irqfd.h
+++ b/include/linux/kvm_irqfd.h
@@ -31,7 +31,7 @@ struct kvm_kernel_irqfd_resampler {
 	/*
 	 * Entry in list of kvm->irqfd.resampler_list.  Use for sharing
 	 * resamplers among irqfds on the same gsi.
-	 * Accessed and modified under kvm->irqfds.resampler_lock
+	 * RCU list modified under kvm->irqfds.resampler_lock
 	 */
 	struct list_head link;
 };