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, 5 insertions, 1 deletions

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index fef88d84cef..bfef604160d 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -227,7 +227,7 @@ static int virtballoon_probe(struct virtio_device *vdev)
 	}
 
 	vb->tell_host_first
-		= vdev->config->feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
+		= virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
 
 	return 0;
 
@@ -259,7 +259,11 @@ static void virtballoon_remove(struct virtio_device *vdev)
 	kfree(vb);
 }
 
+static unsigned int features[] = { VIRTIO_BALLOON_F_MUST_TELL_HOST };
+
 static struct virtio_driver virtio_balloon = {
+	.feature_table = features,
+	.feature_table_size = ARRAY_SIZE(features),
 	.driver.name =	KBUILD_MODNAME,
 	.driver.owner =	THIS_MODULE,
 	.id_table =	id_table,