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 */