virtio: explicit advertisement of driver features

A recent proposed feature addition to the virtio block driver revealed
some flaws in the API: in particular, we assume that feature
negotiation is complete once a driver's probe function returns.

There is nothing in the API to require this, however, and even I
didn't notice when it was violated.

So instead, we require the driver to specify what features it supports
in a table, we can then move the feature negotiation into the virtio
core.  The intersection of device and driver features are presented in
a new 'features' bitmap in the struct virtio_device.

Note that this highlights the difference between Linux unsigned-long
bitmaps where each unsigned long is in native endian, and a
straight-forward little-endian array of bytes.

Drivers can still remove feature bits in their probe routine if they
really have to.

API changes:
- dev->config->feature() no longer gets and acks a feature.
- drivers should advertise their features in the 'feature_table' field
- use virtio_has_feature() for extra sanity when checking feature bits

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 15 insertions, 7 deletions

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index ad43421ab6f1..f926b5ab3d09 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -378,26 +378,26 @@ static int virtnet_probe(struct virtio_device *vdev)
 	SET_NETDEV_DEV(dev, &vdev->dev);
 
 	/* Do we support "hardware" checksums? */
-	if (csum && vdev->config->feature(vdev, VIRTIO_NET_F_CSUM)) {
+	if (csum && virtio_has_feature(vdev, VIRTIO_NET_F_CSUM)) {
 		/* This opens up the world of extra features. */
 		dev->features |= NETIF_F_HW_CSUM|NETIF_F_SG|NETIF_F_FRAGLIST;
-		if (gso && vdev->config->feature(vdev, VIRTIO_NET_F_GSO)) {
+		if (gso && virtio_has_feature(vdev, VIRTIO_NET_F_GSO)) {
 			dev->features |= NETIF_F_TSO | NETIF_F_UFO
 				| NETIF_F_TSO_ECN | NETIF_F_TSO6;
 		}
 		/* Individual feature bits: what can host handle? */
-		if (gso && vdev->config->feature(vdev, VIRTIO_NET_F_HOST_TSO4))
+		if (gso && virtio_has_feature(vdev, VIRTIO_NET_F_HOST_TSO4))
 			dev->features |= NETIF_F_TSO;
-		if (gso && vdev->config->feature(vdev, VIRTIO_NET_F_HOST_TSO6))
+		if (gso && virtio_has_feature(vdev, VIRTIO_NET_F_HOST_TSO6))
 			dev->features |= NETIF_F_TSO6;
-		if (gso && vdev->config->feature(vdev, VIRTIO_NET_F_HOST_ECN))
+		if (gso && virtio_has_feature(vdev, VIRTIO_NET_F_HOST_ECN))
 			dev->features |= NETIF_F_TSO_ECN;
-		if (gso && vdev->config->feature(vdev, VIRTIO_NET_F_HOST_UFO))
+		if (gso && virtio_has_feature(vdev, VIRTIO_NET_F_HOST_UFO))
 			dev->features |= NETIF_F_UFO;
 	}
 
 	/* Configuration may specify what MAC to use.  Otherwise random. */
-	if (vdev->config->feature(vdev, VIRTIO_NET_F_MAC)) {
+	if (virtio_has_feature(vdev, VIRTIO_NET_F_MAC)) {
 		vdev->config->get(vdev,
 				  offsetof(struct virtio_net_config, mac),
 				  dev->dev_addr, dev->addr_len);
@@ -486,7 +486,15 @@ static struct virtio_device_id id_table[] = {
 	{ 0 },
 };
 
+static unsigned int features[] = {
+	VIRTIO_NET_F_CSUM, VIRTIO_NET_F_GSO, VIRTIO_NET_F_MAC,
+	VIRTIO_NET_F_HOST_TSO4, VIRTIO_NET_F_HOST_UFO, VIRTIO_NET_F_HOST_TSO6,
+	VIRTIO_NET_F_HOST_ECN,
+};
+
 static struct virtio_driver virtio_net = {
+	.feature_table = features,
+	.feature_table_size = ARRAY_SIZE(features),
 	.driver.name =	KBUILD_MODNAME,
 	.driver.owner =	THIS_MODULE,
 	.id_table =	id_table,