[RFC PATCH v4 13/25] vfio: Track device use by guest_memfd

["Aneesh Kumar K.V (Arm)" <aneesh.kumar@kernel.org>]

Add gmem_attached to record whether a device supplies guest_memfd
backing, and initialize gmem_lock.

Changing gmem_attached requires both gmem_lock held for write and
dev_set->lock held. This lets device file operations check the flag
under gmem_lock for read, while reset checks use dev_set->lock.

This patch adds the flag and initializes the lock. Later patches set
the flag after successful provider setup and clear it after provider
cleanup.

Cc: Alex Williamson <alex@shazbot.org>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: David Hildenbrand <david@kernel.org>
Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
---
 drivers/vfio/vfio_main.c | 1 +
 include/linux/vfio.h     | 7 +++++++
 2 files changed, 8 insertions(+)

diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
index 4e5d738ccf4b..ff49f2fdb602 100644
--- a/drivers/vfio/vfio_main.c
+++ b/drivers/vfio/vfio_main.c
@@ -290,6 +290,7 @@ static int vfio_init_device(struct vfio_device *device, struct device *dev,
 		return ret;
 	}
 
+	init_rwsem(&device->gmem_lock);
 	device->index = ret;
 	init_completion(&device->comp);
 	device->dev = dev;
diff --git a/include/linux/vfio.h b/include/linux/vfio.h
index 0cc91c6f96d2..2436b8bd600a 100644
--- a/include/linux/vfio.h
+++ b/include/linux/vfio.h
@@ -37,6 +37,13 @@ struct vfio_device_set {
 };
 
 struct vfio_device {
+	/*
+	 * Serializes guest_memfd provider setup and teardown with device file
+	 * operations.
+	 */
+	struct rw_semaphore gmem_lock;
+	/* Updates hold gmem_lock for write and dev_set->lock; either permits reads. */
+	bool gmem_attached;
 	struct device *dev;
 	const struct vfio_device_ops *ops;
 	/*
-- 
2.43.0