1.. SPDX-License-Identifier: GPL-2.0
2
3.. _writing_virtio_drivers:
4
5======================
6Writing Virtio Drivers
7======================
8
9Introduction
10============
11
12This document serves as a basic guideline for driver programmers that
13need to hack a new virtio driver or understand the essentials of the
14existing ones. See :ref:`Virtio on Linux <virtio>` for a general
15overview of virtio.
16
17
18Driver boilerplate
19==================
20
21As a bare minimum, a virtio driver needs to register in the virtio bus
22and configure the virtqueues for the device according to its spec, the
23configuration of the virtqueues in the driver side must match the
24virtqueue definitions in the device. A basic driver skeleton could look
25like this::
26
27	#include <linux/virtio.h>
28	#include <linux/virtio_ids.h>
29	#include <linux/virtio_config.h>
30	#include <linux/module.h>
31
32	/* device private data (one per device) */
33	struct virtio_dummy_dev {
34		struct virtqueue *vq;
35	};
36
37	static void virtio_dummy_recv_cb(struct virtqueue *vq)
38	{
39		struct virtio_dummy_dev *dev = vq->vdev->priv;
40		char *buf;
41		unsigned int len;
42
43		while ((buf = virtqueue_get_buf(dev->vq, &len)) != NULL) {
44			/* process the received data */
45		}
46	}
47
48	static int virtio_dummy_probe(struct virtio_device *vdev)
49	{
50		struct virtio_dummy_dev *dev = NULL;
51
52		/* initialize device data */
53		dev = kzalloc(sizeof(struct virtio_dummy_dev), GFP_KERNEL);
54		if (!dev)
55			return -ENOMEM;
56
57		/* the device has a single virtqueue */
58		dev->vq = virtio_find_single_vq(vdev, virtio_dummy_recv_cb, "input");
59		if (IS_ERR(dev->vq)) {
60			kfree(dev);
61			return PTR_ERR(dev->vq);
62
63		}
64		vdev->priv = dev;
65
66		/* from this point on, the device can notify and get callbacks */
67		virtio_device_ready(vdev);
68
69		return 0;
70	}
71
72	static void virtio_dummy_remove(struct virtio_device *vdev)
73	{
74		struct virtio_dummy_dev *dev = vdev->priv;
75
76		/*
77		 * disable vq interrupts: equivalent to
78		 * vdev->config->reset(vdev)
79		 */
80		virtio_reset_device(vdev);
81
82		/* detach unused buffers */
83		while ((buf = virtqueue_detach_unused_buf(dev->vq)) != NULL) {
84			kfree(buf);
85		}
86
87		/* remove virtqueues */
88		vdev->config->del_vqs(vdev);
89
90		kfree(dev);
91	}
92
93	static const struct virtio_device_id id_table[] = {
94		{ VIRTIO_ID_DUMMY, VIRTIO_DEV_ANY_ID },
95		{ 0 },
96	};
97
98	static struct virtio_driver virtio_dummy_driver = {
99		.driver.name =  KBUILD_MODNAME,
100		.driver.owner = THIS_MODULE,
101		.id_table =     id_table,
102		.probe =        virtio_dummy_probe,
103		.remove =       virtio_dummy_remove,
104	};
105
106	module_virtio_driver(virtio_dummy_driver);
107	MODULE_DEVICE_TABLE(virtio, id_table);
108	MODULE_DESCRIPTION("Dummy virtio driver");
109	MODULE_LICENSE("GPL");
110
111The device id ``VIRTIO_ID_DUMMY`` here is a placeholder, virtio drivers
112should be added only for devices that are defined in the spec, see
113include/uapi/linux/virtio_ids.h. Device ids need to be at least reserved
114in the virtio spec before being added to that file.
115
116If your driver doesn't have to do anything special in its ``init`` and
117``exit`` methods, you can use the module_virtio_driver() helper to
118reduce the amount of boilerplate code.
119
120The ``probe`` method does the minimum driver setup in this case
121(memory allocation for the device data) and initializes the
122virtqueue. virtio_device_ready() is used to enable the virtqueue and to
123notify the device that the driver is ready to manage the device
124("DRIVER_OK"). The virtqueues are anyway enabled automatically by the
125core after ``probe`` returns.
126
127.. kernel-doc:: include/linux/virtio_config.h
128    :identifiers: virtio_device_ready
129
130In any case, the virtqueues need to be enabled before adding buffers to
131them.
132
133Sending and receiving data
134==========================
135
136The virtio_dummy_recv_cb() callback in the code above will be triggered
137when the device notifies the driver after it finishes processing a
138descriptor or descriptor chain, either for reading or writing. However,
139that's only the second half of the virtio device-driver communication
140process, as the communication is always started by the driver regardless
141of the direction of the data transfer.
142
143To configure a buffer transfer from the driver to the device, first you
144have to add the buffers -- packed as `scatterlists` -- to the
145appropriate virtqueue using any of the virtqueue_add_inbuf(),
146virtqueue_add_outbuf() or virtqueue_add_sgs(), depending on whether you
147need to add one input `scatterlist` (for the device to fill in), one
148output `scatterlist` (for the device to consume) or multiple
149`scatterlists`, respectively. Then, once the virtqueue is set up, a call
150to virtqueue_kick() sends a notification that will be serviced by the
151hypervisor that implements the device::
152
153	struct scatterlist sg[1];
154	sg_init_one(sg, buffer, BUFLEN);
155	virtqueue_add_inbuf(dev->vq, sg, 1, buffer, GFP_ATOMIC);
156	virtqueue_kick(dev->vq);
157
158.. kernel-doc:: drivers/virtio/virtio_ring.c
159    :identifiers: virtqueue_add_inbuf
160
161.. kernel-doc:: drivers/virtio/virtio_ring.c
162    :identifiers: virtqueue_add_outbuf
163
164.. kernel-doc:: drivers/virtio/virtio_ring.c
165    :identifiers: virtqueue_add_sgs
166
167Then, after the device has read or written the buffers prepared by the
168driver and notifies it back, the driver can call virtqueue_get_buf() to
169read the data produced by the device (if the virtqueue was set up with
170input buffers) or simply to reclaim the buffers if they were already
171consumed by the device:
172
173.. kernel-doc:: drivers/virtio/virtio_ring.c
174    :identifiers: virtqueue_get_buf_ctx
175
176The virtqueue callbacks can be disabled and re-enabled using the
177virtqueue_disable_cb() and the family of virtqueue_enable_cb() functions
178respectively. See drivers/virtio/virtio_ring.c for more details:
179
180.. kernel-doc:: drivers/virtio/virtio_ring.c
181    :identifiers: virtqueue_disable_cb
182
183.. kernel-doc:: drivers/virtio/virtio_ring.c
184    :identifiers: virtqueue_enable_cb
185
186But note that some spurious callbacks can still be triggered under
187certain scenarios. The way to disable callbacks reliably is to reset the
188device or the virtqueue (virtio_reset_device()).
189
190
191References
192==========
193
194_`[1]` Virtio Spec v1.2:
195https://docs.oasis-open.org/virtio/virtio/v1.2/virtio-v1.2.html
196
197Check for later versions of the spec as well.
198