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>

3 files changed, 56 insertions, 18 deletions

diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
index b535483bc556..13866789b356 100644
--- a/drivers/virtio/virtio.c
+++ b/drivers/virtio/virtio.c
@@ -80,19 +80,51 @@ static void add_status(struct virtio_device *dev, unsigned status)
 	dev->config->set_status(dev, dev->config->get_status(dev) | status);
 }
 
+void virtio_check_driver_offered_feature(const struct virtio_device *vdev,
+					 unsigned int fbit)
+{
+	unsigned int i;
+	struct virtio_driver *drv = container_of(vdev->dev.driver,
+						 struct virtio_driver, driver);
+
+	for (i = 0; i < drv->feature_table_size; i++)
+		if (drv->feature_table[i] == fbit)
+			return;
+	BUG();
+}
+EXPORT_SYMBOL_GPL(virtio_check_driver_offered_feature);
+
 static int virtio_dev_probe(struct device *_d)
 {
-	int err;
+	int err, i;
 	struct virtio_device *dev = container_of(_d,struct virtio_device,dev);
 	struct virtio_driver *drv = container_of(dev->dev.driver,
 						 struct virtio_driver, driver);
+	u32 device_features;
 
+	/* We have a driver! */
 	add_status(dev, VIRTIO_CONFIG_S_DRIVER);
+
+	/* Figure out what features the device supports. */
+	device_features = dev->config->get_features(dev);
+
+	/* Features supported by both device and driver into dev->features. */
+	memset(dev->features, 0, sizeof(dev->features));
+	for (i = 0; i < drv->feature_table_size; i++) {
+		unsigned int f = drv->feature_table[i];
+		BUG_ON(f >= 32);
+		if (device_features & (1 << f))
+			set_bit(f, dev->features);
+	}
+
 	err = drv->probe(dev);
 	if (err)
 		add_status(dev, VIRTIO_CONFIG_S_FAILED);
-	else
+	else {
 		add_status(dev, VIRTIO_CONFIG_S_DRIVER_OK);
+		/* They should never have set feature bits beyond 32 */
+		dev->config->set_features(dev, dev->features[0]);
+	}
 	return err;
 }
 
@@ -114,6 +146,8 @@ static int virtio_dev_remove(struct device *_d)
 
 int register_virtio_driver(struct virtio_driver *driver)
 {
+	/* Catch this early. */
+	BUG_ON(driver->feature_table_size && !driver->feature_table);
 	driver->driver.bus = &virtio_bus;
 	driver->driver.probe = virtio_dev_probe;
 	driver->driver.remove = virtio_dev_remove;
diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index fef88d84cef6..bfef604160d1 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,
diff --git a/drivers/virtio/virtio_pci.c b/drivers/virtio/virtio_pci.c
index de102a614e97..27e9fc9117cd 100644
--- a/drivers/virtio/virtio_pci.c
+++ b/drivers/virtio/virtio_pci.c
@@ -87,23 +87,22 @@ static struct virtio_pci_device *to_vp_device(struct virtio_device *vdev)
 	return container_of(vdev, struct virtio_pci_device, vdev);
 }
 
-/* virtio config->feature() implementation */
-static bool vp_feature(struct virtio_device *vdev, unsigned bit)
+/* virtio config->get_features() implementation */
+static u32 vp_get_features(struct virtio_device *vdev)
+{
+	struct virtio_pci_device *vp_dev = to_vp_device(vdev);
+
+	/* When someone needs more than 32 feature bits, we'll need to
+	 * steal a bit to indicate that the rest are somewhere else. */
+	return ioread32(vp_dev->ioaddr + VIRTIO_PCI_HOST_FEATURES);
+}
+
+/* virtio config->set_features() implementation */
+static void vp_set_features(struct virtio_device *vdev, u32 features)
 {
 	struct virtio_pci_device *vp_dev = to_vp_device(vdev);
-	u32 mask;
-
-	/* Since this function is supposed to have the side effect of
-	 * enabling a queried feature, we simulate that by doing a read
-	 * from the host feature bitmask and then writing to the guest
-	 * feature bitmask */
-	mask = ioread32(vp_dev->ioaddr + VIRTIO_PCI_HOST_FEATURES);
-	if (mask & (1 << bit)) {
-		mask |= (1 << bit);
-		iowrite32(mask, vp_dev->ioaddr + VIRTIO_PCI_GUEST_FEATURES);
-	}
 
-	return !!(mask & (1 << bit));
+	iowrite32(features, vp_dev->ioaddr + VIRTIO_PCI_GUEST_FEATURES);
 }
 
 /* virtio config->get() implementation */
@@ -293,7 +292,6 @@ static void vp_del_vq(struct virtqueue *vq)
 }
 
 static struct virtio_config_ops virtio_pci_config_ops = {
-	.feature	= vp_feature,
 	.get		= vp_get,
 	.set		= vp_set,
 	.get_status	= vp_get_status,
@@ -301,6 +299,8 @@ static struct virtio_config_ops virtio_pci_config_ops = {
 	.reset		= vp_reset,
 	.find_vq	= vp_find_vq,
 	.del_vq		= vp_del_vq,
+	.get_features	= vp_get_features,
+	.set_features	= vp_set_features,
 };
 
 /* the PCI probing function */