virtio: add debugfs infrastructure to allow to debug virtio features

Currently there is no way for user to set what features the driver
should obey or not, it is hard wired in the code.

In order to be able to debug the device behavior in case some feature is
disabled, introduce a debugfs infrastructure with couple of files
allowing user to see what features the device advertises and
to set filter for features used by driver.

Example:
$cat /sys/bus/virtio/devices/virtio0/features
1110010111111111111101010000110010000000100000000000000000000000
$ echo "5" >/sys/kernel/debug/virtio/virtio0/filter_feature_add
$ cat /sys/kernel/debug/virtio/virtio0/filter_features
5
$ echo "virtio0" > /sys/bus/virtio/drivers/virtio_net/unbind
$ echo "virtio0" > /sys/bus/virtio/drivers/virtio_net/bind
$ cat /sys/bus/virtio/devices/virtio0/features
1110000111111111111101010000110010000000100000000000000000000000

Note that sysfs "features" now already exists, this patch does not
touch it.
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/virtio/Kconfig

@@ -178,4 +178,14 @@ config VIRTIO_DMA_SHARED_BUFFER
 	 This option adds a flavor of dma buffers that are backed by
 	 virtio resources.
 
+config VIRTIO_DEBUG
+	bool "Debug facilities"
+	depends on VIRTIO
+	help
+	  Enable this to expose debug facilities over debugfs.
+	  This allows to debug features, to see what features the device
+	  advertises and to set filter for features used by driver.
+
+	  If unsure, say N.
+
 endif # VIRTIO_MENU

drivers/virtio/Makefile

@@ -13,3 +13,4 @@ obj-$(CONFIG_VIRTIO_INPUT) += virtio_input.o
 obj-$(CONFIG_VIRTIO_VDPA) += virtio_vdpa.o
 obj-$(CONFIG_VIRTIO_MEM) += virtio_mem.o
 obj-$(CONFIG_VIRTIO_DMA_SHARED_BUFFER) += virtio_dma_buf.o
+obj-$(CONFIG_VIRTIO_DEBUG) += virtio_debug.o

drivers/virtio/virtio.c

@@ -274,6 +274,9 @@ static int virtio_dev_probe(struct device *_d)
 	else
 		dev->features = driver_features_legacy & device_features;
 
+	/* When debugging, user may filter some features by hand. */
+	virtio_debug_device_filter_features(dev);
+
 	/* Transport features always preserved to pass to finalize_features. */
 	for (i = VIRTIO_TRANSPORT_F_START; i < VIRTIO_TRANSPORT_F_END; i++)
 		if (device_features & (1ULL << i))
@@ -465,6 +468,8 @@ int register_virtio_device(struct virtio_device *dev)
 	/* Acknowledge that we've seen the device. */
 	virtio_add_status(dev, VIRTIO_CONFIG_S_ACKNOWLEDGE);
 
+	virtio_debug_device_init(dev);
+
 	/*
 	 * device_add() causes the bus infrastructure to look for a matching
 	 * driver.
@@ -496,6 +501,7 @@ void unregister_virtio_device(struct virtio_device *dev)
 	int index = dev->index; /* save for after device release */
 
 	device_unregister(&dev->dev);
+	virtio_debug_device_exit(dev);
 	ida_free(&virtio_index_ida, index);
 }
 EXPORT_SYMBOL_GPL(unregister_virtio_device);
@@ -590,11 +596,13 @@ static int virtio_init(void)
 {
 	if (bus_register(&virtio_bus) != 0)
 		panic("virtio bus registration failed");
+	virtio_debug_init();
 	return 0;
 }
 
 static void __exit virtio_exit(void)
 {
+	virtio_debug_exit();
 	bus_unregister(&virtio_bus);
 	ida_destroy(&virtio_index_ida);
 }

drivers/virtio/virtio_debug.c

+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include <linux/virtio.h>
+#include <linux/virtio_config.h>
+#include <linux/debugfs.h>
+
+static struct dentry *virtio_debugfs_dir;
+
+static int virtio_debug_device_features_show(struct seq_file *s, void *data)
+{
+	struct virtio_device *dev = s->private;
+	u64 device_features;
+	unsigned int i;
+
+	device_features = dev->config->get_features(dev);
+	for (i = 0; i < BITS_PER_LONG_LONG; i++) {
+		if (device_features & (1ULL << i))
+			seq_printf(s, "%u\n", i);
+	}
+	return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(virtio_debug_device_features);
+
+static int virtio_debug_filter_features_show(struct seq_file *s, void *data)
+{
+	struct virtio_device *dev = s->private;
+	unsigned int i;
+
+	for (i = 0; i < BITS_PER_LONG_LONG; i++) {
+		if (dev->debugfs_filter_features & (1ULL << i))
+			seq_printf(s, "%u\n", i);
+	}
+	return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(virtio_debug_filter_features);
+
+static int virtio_debug_filter_features_clear(void *data, u64 val)
+{
+	struct virtio_device *dev = data;
+
+	if (val == 1)
+		dev->debugfs_filter_features = 0;
+	return 0;
+}
+
+DEFINE_DEBUGFS_ATTRIBUTE(virtio_debug_filter_features_clear_fops, NULL,
+			 virtio_debug_filter_features_clear, "%llu\n");
+
+static int virtio_debug_filter_feature_add(void *data, u64 val)
+{
+	struct virtio_device *dev = data;
+
+	if (val >= BITS_PER_LONG_LONG)
+		return -EINVAL;
+	dev->debugfs_filter_features |= BIT_ULL_MASK(val);
+	return 0;
+}
+
+DEFINE_DEBUGFS_ATTRIBUTE(virtio_debug_filter_feature_add_fops, NULL,
+			 virtio_debug_filter_feature_add, "%llu\n");
+
+static int virtio_debug_filter_feature_del(void *data, u64 val)
+{
+	struct virtio_device *dev = data;
+
+	if (val >= BITS_PER_LONG_LONG)
+		return -EINVAL;
+	dev->debugfs_filter_features &= ~BIT_ULL_MASK(val);
+	return 0;
+}
+
+DEFINE_DEBUGFS_ATTRIBUTE(virtio_debug_filter_feature_del_fops, NULL,
+			 virtio_debug_filter_feature_del, "%llu\n");
+
+void virtio_debug_device_init(struct virtio_device *dev)
+{
+	dev->debugfs_dir = debugfs_create_dir(dev_name(&dev->dev),
+					      virtio_debugfs_dir);
+	debugfs_create_file("device_features", 0400, dev->debugfs_dir, dev,
+			    &virtio_debug_device_features_fops);
+	debugfs_create_file("filter_features", 0400, dev->debugfs_dir, dev,
+			    &virtio_debug_filter_features_fops);
+	debugfs_create_file("filter_features_clear", 0200, dev->debugfs_dir, dev,
+			    &virtio_debug_filter_features_clear_fops);
+	debugfs_create_file("filter_feature_add", 0200, dev->debugfs_dir, dev,
+			    &virtio_debug_filter_feature_add_fops);
+	debugfs_create_file("filter_feature_del", 0200, dev->debugfs_dir, dev,
+			    &virtio_debug_filter_feature_del_fops);
+}
+EXPORT_SYMBOL_GPL(virtio_debug_device_init);
+
+void virtio_debug_device_filter_features(struct virtio_device *dev)
+{
+	dev->features &= ~dev->debugfs_filter_features;
+}
+EXPORT_SYMBOL_GPL(virtio_debug_device_filter_features);
+
+void virtio_debug_device_exit(struct virtio_device *dev)
+{
+	debugfs_remove_recursive(dev->debugfs_dir);
+}
+EXPORT_SYMBOL_GPL(virtio_debug_device_exit);
+
+void virtio_debug_init(void)
+{
+	virtio_debugfs_dir = debugfs_create_dir("virtio", NULL);
+}
+EXPORT_SYMBOL_GPL(virtio_debug_init);
+
+void virtio_debug_exit(void)
+{
+	debugfs_remove_recursive(virtio_debugfs_dir);
+}
+EXPORT_SYMBOL_GPL(virtio_debug_exit);

include/linux/virtio.h

@@ -126,6 +126,8 @@ struct virtio_admin_cmd {
  * @vqs: the list of virtqueues for this device.
  * @features: the features supported by both driver and device.
  * @priv: private pointer for the driver's use.
+ * @debugfs_dir: debugfs directory entry.
+ * @debugfs_filter_features: features to be filtered set by debugfs.
  */
 struct virtio_device {
 	int index;
@@ -141,6 +143,10 @@ struct virtio_device {
 	struct list_head vqs;
 	u64 features;
 	void *priv;
+#ifdef CONFIG_VIRTIO_DEBUG
+	struct dentry *debugfs_dir;
+	u64 debugfs_filter_features;
+#endif
 };
 
 #define dev_to_virtio(_dev)	container_of_const(_dev, struct virtio_device, dev)
@@ -237,4 +243,33 @@ void virtqueue_dma_sync_single_range_for_cpu(struct virtqueue *_vq, dma_addr_t a
 void virtqueue_dma_sync_single_range_for_device(struct virtqueue *_vq, dma_addr_t addr,
 						unsigned long offset, size_t size,
 						enum dma_data_direction dir);
+
+#ifdef CONFIG_VIRTIO_DEBUG
+void virtio_debug_device_init(struct virtio_device *dev);
+void virtio_debug_device_exit(struct virtio_device *dev);
+void virtio_debug_device_filter_features(struct virtio_device *dev);
+void virtio_debug_init(void);
+void virtio_debug_exit(void);
+#else
+static inline void virtio_debug_device_init(struct virtio_device *dev)
+{
+}
+
+static inline void virtio_debug_device_exit(struct virtio_device *dev)
+{
+}
+
+static inline void virtio_debug_device_filter_features(struct virtio_device *dev)
+{
+}
+
+static inline void virtio_debug_init(void)
+{
+}
+
+static inline void virtio_debug_exit(void)
+{
+}
+#endif
+
 #endif /* _LINUX_VIRTIO_H */