virtio: add legacy feature table support

virtio-blk has some legacy feature bits that modern drivers
must not negotiate, but are needed for old legacy hosts
(that e.g. don't support virtio-scsi).
Allow a separate legacy feature table for such cases.

Signed-off-by: Michael S. Tsirkin <[e-mail]>
Reviewed-by: Cornelia Huck <[e-mail]>

drivers/virtio/virtio.c

···
 	for (i = 0; i < drv->feature_table_size; i++)
 		if (drv->feature_table[i] == fbit)
 			return;
+
+	if (drv->feature_table_legacy) {
+		for (i = 0; i < drv->feature_table_size_legacy; i++)
+			if (drv->feature_table_legacy[i] == fbit)
+				return;
+	}
+
 	BUG();
 }
 EXPORT_SYMBOL_GPL(virtio_check_driver_offered_feature);
···
 	struct virtio_driver *drv = drv_to_virtio(dev->dev.driver);
 	u64 device_features;
 	u64 driver_features;
+	u64 driver_features_legacy;
 	unsigned status;
 
 	/* We have a driver! */
···
 		driver_features |= (1ULL << f);
 	}
 
-	dev->features = driver_features & device_features;
+	/* Some drivers have a separate feature table for virtio v1.0 */
+	if (drv->feature_table_legacy) {
+		driver_features_legacy = 0;
+		for (i = 0; i < drv->feature_table_size_legacy; i++) {
+			unsigned int f = drv->feature_table_legacy[i];
+			BUG_ON(f >= 64);
+			driver_features_legacy |= (1ULL << f);
+		}
+	} else {
+		driver_features_legacy = driver_features;
+	}
+
+	if (driver_features & device_features & (1ULL << VIRTIO_F_VERSION_1))
+		dev->features = driver_features & device_features;
+	else
+		dev->features = driver_features_legacy & device_features;
 
 	/* Transport features always preserved to pass to finalize_features. */
 	for (i = VIRTIO_TRANSPORT_F_START; i < VIRTIO_TRANSPORT_F_END; i++)

include/linux/virtio.h

···
  * @id_table: the ids serviced by this driver.
  * @feature_table: an array of feature numbers supported by this driver.
  * @feature_table_size: number of entries in the feature table array.
+ * @feature_table_legacy: same as feature_table but when working in legacy mode.
+ * @feature_table_size_legacy: number of entries in feature table legacy array.
  * @probe: the function to call when a device is found.  Returns 0 or -errno.
  * @remove: the function to call when a device is removed.
  * @config_changed: optional function to call when the device configuration
···
 	const struct virtio_device_id *id_table;
 	const unsigned int *feature_table;
 	unsigned int feature_table_size;
+	const unsigned int *feature_table_legacy;
+	unsigned int feature_table_size_legacy;
 	int (*probe)(struct virtio_device *dev);
 	void (*scan)(struct virtio_device *dev);
 	void (*remove)(struct virtio_device *dev);