kvm/vfio: avoid bouncing the mutex when adding and deleting groups

Stop taking kv->lock mutex in kvm_vfio_update_coherency() and instead
call it with this mutex held: the callers of the function usually
already have it taken (and released) before calling
kvm_vfio_update_coherency(). This avoid bouncing the lock up and down.

The exception is kvm_vfio_release() where we do not take the lock, but
it is being executed when the very last reference to kvm_device is being
dropped, so there are no concerns about concurrency.

Suggested-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20230714224538.404793-2-dmitry.torokhov@gmail.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

virt/kvm/vfio.c

@@ -123,8 +123,6 @@ static void kvm_vfio_update_coherency(struct kvm_device *dev)
 	bool noncoherent = false;
 	struct kvm_vfio_file *kvf;
 
-	mutex_lock(&kv->lock);
-
 	list_for_each_entry(kvf, &kv->file_list, node) {
 		if (!kvm_vfio_file_enforced_coherent(kvf->file)) {
 			noncoherent = true;
@@ -140,16 +138,14 @@ static void kvm_vfio_update_coherency(struct kvm_device *dev)
 		else
 			kvm_arch_unregister_noncoherent_dma(dev->kvm);
 	}
-
-	mutex_unlock(&kv->lock);
 }
 
 static int kvm_vfio_file_add(struct kvm_device *dev, unsigned int fd)
 {
 	struct kvm_vfio *kv = dev->private;
 	struct kvm_vfio_file *kvf;
 	struct file *filp;
-	int ret;
+	int ret = 0;
 
 	filp = fget(fd);
 	if (!filp)
@@ -158,38 +154,34 @@ static int kvm_vfio_file_add(struct kvm_device *dev, unsigned int fd)
 	/* Ensure the FD is a vfio FD. */
 	if (!kvm_vfio_file_is_valid(filp)) {
 		ret = -EINVAL;
-		goto err_fput;
+		goto out_fput;
 	}
 
 	mutex_lock(&kv->lock);
 
 	list_for_each_entry(kvf, &kv->file_list, node) {
 		if (kvf->file == filp) {
 			ret = -EEXIST;
-			goto err_unlock;
+			goto out_unlock;
 		}
 	}
 
 	kvf = kzalloc(sizeof(*kvf), GFP_KERNEL_ACCOUNT);
 	if (!kvf) {
 		ret = -ENOMEM;
-		goto err_unlock;
+		goto out_unlock;
 	}
 
-	kvf->file = filp;
+	kvf->file = get_file(filp);
 	list_add_tail(&kvf->node, &kv->file_list);
 
 	kvm_arch_start_assignment(dev->kvm);
 	kvm_vfio_file_set_kvm(kvf->file, dev->kvm);
-
-	mutex_unlock(&kv->lock);
-
 	kvm_vfio_update_coherency(dev);
 
-	return 0;
-err_unlock:
+out_unlock:
 	mutex_unlock(&kv->lock);
-err_fput:
+out_fput:
 	fput(filp);
 	return ret;
 }
@@ -225,12 +217,12 @@ static int kvm_vfio_file_del(struct kvm_device *dev, unsigned int fd)
 		break;
 	}
 
+	kvm_vfio_update_coherency(dev);
+
 	mutex_unlock(&kv->lock);
 
 	fdput(f);
 
-	kvm_vfio_update_coherency(dev);
-
 	return ret;
 }