virtio: Extend the admin command to include the result size

Extend the admin command by incorporating a result size field.

This allows higher layers to determine the actual result size from the
backend when this information is not included in the result_sg.

The additional information introduced here will be used in subsequent
patches of this series.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Link: https://lore.kernel.org/r/20241113115200.209269-3-yishaih@nvidia.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/virtio/virtio_pci_modern.c

@@ -64,8 +64,10 @@ void vp_modern_avq_done(struct virtqueue *vq)
 	spin_lock_irqsave(&admin_vq->lock, flags);
 	do {
 		virtqueue_disable_cb(vq);
-		while ((cmd = virtqueue_get_buf(vq, &len)))
+		while ((cmd = virtqueue_get_buf(vq, &len))) {
+			cmd->result_sg_size = len;
 			complete(&cmd->completion);
+		}
 	} while (!virtqueue_enable_cb(vq));
 	spin_unlock_irqrestore(&admin_vq->lock, flags);
 }

include/linux/virtio.h

@@ -111,6 +111,7 @@ struct virtio_admin_cmd {
 	struct scatterlist *data_sg;
 	struct scatterlist *result_sg;
 	struct completion completion;
+	u32 result_sg_size;
 	int ret;
 };