vdpa_sim: add get_config callback in vdpasim_dev_attr

The get_config callback can be used by the device to fill the
config structure.
The callback will be invoked in vdpasim_get_config() before copying
bytes into caller buffer.

Move vDPA-net config updates from vdpasim_set_features() in the
new vdpasim_net_get_config() callback.
This is safe since in vdpa_get_config() we already check that
.set_features() callback is called before .get_config().
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20201215144256.155342-13-sgarzare@redhat.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vdpa/vdpa_sim/vdpa_sim.c

@@ -68,6 +68,8 @@ struct vdpasim_virtqueue {
 #define VDPASIM_NET_FEATURES	(VDPASIM_FEATURES | \
 				 (1ULL << VIRTIO_NET_F_MAC))
 
+struct vdpasim;
+
 struct vdpasim_dev_attr {
 	u64 supported_features;
 	size_t config_size;
@@ -75,6 +77,7 @@ struct vdpasim_dev_attr {
 	u32 id;
 
 	work_func_t work_fn;
+	void (*get_config)(struct vdpasim *vdpasim, void *config);
 };
 
 /* State of each vdpasim device */
@@ -530,8 +533,6 @@ static u64 vdpasim_get_features(struct vdpa_device *vdpa)
 static int vdpasim_set_features(struct vdpa_device *vdpa, u64 features)
 {
 	struct vdpasim *vdpasim = vdpa_to_sim(vdpa);
-	struct virtio_net_config *config =
-		(struct virtio_net_config *)vdpasim->config;
 
 	/* DMA mapping must be done by driver */
 	if (!(features & (1ULL << VIRTIO_F_ACCESS_PLATFORM)))
@@ -539,16 +540,6 @@ static int vdpasim_set_features(struct vdpa_device *vdpa, u64 features)
 
 	vdpasim->features = features & vdpasim->dev_attr.supported_features;
 
-	/* We generally only know whether guest is using the legacy interface
-	 * here, so generally that's the earliest we can set config fields.
-	 * Note: We actually require VIRTIO_F_ACCESS_PLATFORM above which
-	 * implies VIRTIO_F_VERSION_1, but let's not try to be clever here.
-	 */
-
-	config->mtu = cpu_to_vdpasim16(vdpasim, 1500);
-	config->status = cpu_to_vdpasim16(vdpasim, VIRTIO_NET_S_LINK_UP);
-	memcpy(config->mac, macaddr_buf, ETH_ALEN);
-
 	return 0;
 }
 
@@ -603,8 +594,13 @@ static void vdpasim_get_config(struct vdpa_device *vdpa, unsigned int offset,
 {
 	struct vdpasim *vdpasim = vdpa_to_sim(vdpa);
 
-	if (offset + len < vdpasim->dev_attr.config_size)
-		memcpy(buf, vdpasim->config + offset, len);
+	if (offset + len > vdpasim->dev_attr.config_size)
+		return;
+
+	if (vdpasim->dev_attr.get_config)
+		vdpasim->dev_attr.get_config(vdpasim, vdpasim->config);
+
+	memcpy(buf, vdpasim->config + offset, len);
 }
 
 static void vdpasim_set_config(struct vdpa_device *vdpa, unsigned int offset,
@@ -747,6 +743,16 @@ static const struct vdpa_config_ops vdpasim_batch_config_ops = {
 	.free                   = vdpasim_free,
 };
 
+static void vdpasim_net_get_config(struct vdpasim *vdpasim, void *config)
+{
+	struct virtio_net_config *net_config =
+		(struct virtio_net_config *)config;
+
+	net_config->mtu = cpu_to_vdpasim16(vdpasim, 1500);
+	net_config->status = cpu_to_vdpasim16(vdpasim, VIRTIO_NET_S_LINK_UP);
+	memcpy(net_config->mac, macaddr_buf, ETH_ALEN);
+}
+
 static int __init vdpasim_dev_init(void)
 {
 	struct vdpasim_dev_attr dev_attr = {};
@@ -755,6 +761,7 @@ static int __init vdpasim_dev_init(void)
 	dev_attr.supported_features = VDPASIM_NET_FEATURES;
 	dev_attr.nvqs = VDPASIM_VQ_NUM;
 	dev_attr.config_size = sizeof(struct virtio_net_config);
+	dev_attr.get_config = vdpasim_net_get_config;
 	dev_attr.work_fn = vdpasim_net_work;
 
 	vdpasim_dev = vdpasim_create(&dev_attr);