virtio: Add get_shm_region method

Virtio defines 'shared memory regions' that provide a continuously
shared region between the host and guest.

Provide a method to find a particular region on a device.

Signed-off-by: Sebastien Boeuf <[e-mail]>
Signed-off-by: Dr. David Alan Gilbert <[e-mail]>
Acked-by: Michael S. Tsirkin <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: "Michael S. Tsirkin" <[e-mail]>
Signed-off-by: Miklos Szeredi <[e-mail]>

include/linux/virtio_config.h

···
 
 struct irq_affinity;
 
+struct virtio_shm_region {
+	u64 addr;
+	u64 len;
+};
+
 /**
  * virtio_config_ops - operations for configuring a virtio device
  * Note: Do not assume that a transport implements all of the operations
···
  *      the caller can then copy.
  * @set_vq_affinity: set the affinity for a virtqueue (optional).
  * @get_vq_affinity: get the affinity for a virtqueue (optional).
+ * @get_shm_region: get a shared memory region based on the index.
  */
 typedef void vq_callback_t(struct virtqueue *);
 struct virtio_config_ops {
···
 			       const struct cpumask *cpu_mask);
 	const struct cpumask *(*get_vq_affinity)(struct virtio_device *vdev,
 			int index);
+	bool (*get_shm_region)(struct virtio_device *vdev,
+			       struct virtio_shm_region *region, u8 id);
 };
 
 /* If driver didn't advertise the feature, it will never appear. */
···
 	if (vdev->config->set_vq_affinity)
 		return vdev->config->set_vq_affinity(vq, cpu_mask);
 	return 0;
+}
+
+static inline
+bool virtio_get_shm_region(struct virtio_device *vdev,
+			   struct virtio_shm_region *region, u8 id)
+{
+	if (!vdev->config->get_shm_region)
+		return false;
+	return vdev->config->get_shm_region(vdev, region, id);
 }
 
 static inline bool virtio_is_little_endian(struct virtio_device *vdev)