virtio: Formally reserve bits 28-31 to be 'transport' features.

We assign feature bits as required, but it makes sense to reserve some
for the particular transport, rather than the particular device.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/virtio/virtio.c

@@ -113,6 +113,11 @@ static int virtio_dev_probe(struct device *_d)
 			set_bit(f, dev->features);
 	}
 
+	/* Transport features are always preserved to pass to set_features. */
+	for (i = VIRTIO_TRANSPORT_F_START; i < VIRTIO_TRANSPORT_F_END; i++)
+		if (device_features & (1 << i))
+			set_bit(i, dev->features);
+
 	err = drv->probe(dev);
 	if (err)
 		add_status(dev, VIRTIO_CONFIG_S_FAILED);

include/linux/virtio_config.h

@@ -18,6 +18,12 @@
 /* We've given up on this device. */
 #define VIRTIO_CONFIG_S_FAILED		0x80
 
+/* Some virtio feature bits (currently bits 28 through 31) are reserved for the
+ * transport being used (eg. virtio_ring), the rest are per-device feature
+ * bits. */
+#define VIRTIO_TRANSPORT_F_START	28
+#define VIRTIO_TRANSPORT_F_END		32
+
 /* Do we get callbacks when the ring is completely used, even if we've
  * suppressed them? */
 #define VIRTIO_F_NOTIFY_ON_EMPTY	24