vdpa_sim: add set_config callback in vdpasim_dev_attr

The set_config callback can be used by the device to parse the
config structure modified by the driver.

The callback will be invoked, if set, in vdpasim_set_config() after
copying bytes from caller buffer into vdpasim->config buffer.
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20201215144256.155342-14-sgarzare@redhat.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vdpa/vdpa_sim/vdpa_sim.c

@@ -78,6 +78,7 @@ struct vdpasim_dev_attr {
 
 	work_func_t work_fn;
 	void (*get_config)(struct vdpasim *vdpasim, void *config);
+	void (*set_config)(struct vdpasim *vdpasim, const void *config);
 };
 
 /* State of each vdpasim device */
@@ -606,7 +607,15 @@ static void vdpasim_get_config(struct vdpa_device *vdpa, unsigned int offset,
 static void vdpasim_set_config(struct vdpa_device *vdpa, unsigned int offset,
 			     const void *buf, unsigned int len)
 {
-	/* No writable config supportted by vdpasim */
+	struct vdpasim *vdpasim = vdpa_to_sim(vdpa);
+
+	if (offset + len > vdpasim->dev_attr.config_size)
+		return;
+
+	memcpy(vdpasim->config + offset, buf, len);
+
+	if (vdpasim->dev_attr.set_config)
+		vdpasim->dev_attr.set_config(vdpasim, vdpasim->config);
 }
 
 static u32 vdpasim_get_generation(struct vdpa_device *vdpa)