1 .. SPDX-License-Identifier: GPL-2.0
3 .. _writing_virtio_drivers:
12 This document serves as a basic guideline for driver programmers that
13 need to hack a new virtio driver or understand the essentials of the
14 existing ones. See :ref:`Virtio on Linux <virtio>` for a general
21 As a bare minimum, a virtio driver needs to register in the virtio bus
22 and configure the virtqueues for the device according to its spec, the
23 configuration of the virtqueues in the driver side must match the
24 virtqueue definitions in the device. A basic driver skeleton could look
27 #include <linux/virtio.h>
28 #include <linux/virtio_ids.h>
29 #include <linux/virtio_config.h>
30 #include <linux/module.h>
32 /* device private data (one per device) */
33 struct virtio_dummy_dev {
37 static void virtio_dummy_recv_cb(struct virtqueue *vq)
39 struct virtio_dummy_dev *dev = vq->vdev->priv;
43 while ((buf = virtqueue_get_buf(dev->vq, &len)) != NULL) {
44 /* process the received data */
48 static int virtio_dummy_probe(struct virtio_device *vdev)
50 struct virtio_dummy_dev *dev = NULL;
52 /* initialize device data */
53 dev = kzalloc(sizeof(struct virtio_dummy_dev), GFP_KERNEL);
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)) {
61 return PTR_ERR(dev->vq);
66 /* from this point on, the device can notify and get callbacks */
67 virtio_device_ready(vdev);
72 static void virtio_dummy_remove(struct virtio_device *vdev)
74 struct virtio_dummy_dev *dev = vdev->priv;
77 * disable vq interrupts: equivalent to
78 * vdev->config->reset(vdev)
80 virtio_reset_device(vdev);
82 /* detach unused buffers */
83 while ((buf = virtqueue_detach_unused_buf(dev->vq)) != NULL) {
87 /* remove virtqueues */
88 vdev->config->del_vqs(vdev);
93 static const struct virtio_device_id id_table[] = {
94 { VIRTIO_ID_DUMMY, VIRTIO_DEV_ANY_ID },
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,
106 module_virtio_driver(virtio_dummy_driver);
107 MODULE_DEVICE_TABLE(virtio, id_table);
108 MODULE_DESCRIPTION("Dummy virtio driver");
109 MODULE_LICENSE("GPL");
111 The device id ``VIRTIO_ID_DUMMY`` here is a placeholder, virtio drivers
112 should be added only for devices that are defined in the spec, see
113 include/uapi/linux/virtio_ids.h. Device ids need to be at least reserved
114 in the virtio spec before being added to that file.
116 If 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
118 reduce the amount of boilerplate code.
120 The ``probe`` method does the minimum driver setup in this case
121 (memory allocation for the device data) and initializes the
122 virtqueue. virtio_device_ready() is used to enable the virtqueue and to
123 notify the device that the driver is ready to manage the device
124 ("DRIVER_OK"). The virtqueues are anyway enabled automatically by the
125 core after ``probe`` returns.
127 .. kernel-doc:: include/linux/virtio_config.h
128 :identifiers: virtio_device_ready
130 In any case, the virtqueues need to be enabled before adding buffers to
133 Sending and receiving data
134 ==========================
136 The virtio_dummy_recv_cb() callback in the code above will be triggered
137 when the device notifies the driver after it finishes processing a
138 descriptor or descriptor chain, either for reading or writing. However,
139 that's only the second half of the virtio device-driver communication
140 process, as the communication is always started by the driver regardless
141 of the direction of the data transfer.
143 To configure a buffer transfer from the driver to the device, first you
144 have to add the buffers -- packed as `scatterlists` -- to the
145 appropriate virtqueue using any of the virtqueue_add_inbuf(),
146 virtqueue_add_outbuf() or virtqueue_add_sgs(), depending on whether you
147 need to add one input `scatterlist` (for the device to fill in), one
148 output `scatterlist` (for the device to consume) or multiple
149 `scatterlists`, respectively. Then, once the virtqueue is set up, a call
150 to virtqueue_kick() sends a notification that will be serviced by the
151 hypervisor that implements the device::
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);
158 .. kernel-doc:: drivers/virtio/virtio_ring.c
159 :identifiers: virtqueue_add_inbuf
161 .. kernel-doc:: drivers/virtio/virtio_ring.c
162 :identifiers: virtqueue_add_outbuf
164 .. kernel-doc:: drivers/virtio/virtio_ring.c
165 :identifiers: virtqueue_add_sgs
167 Then, after the device has read or written the buffers prepared by the
168 driver and notifies it back, the driver can call virtqueue_get_buf() to
169 read the data produced by the device (if the virtqueue was set up with
170 input buffers) or simply to reclaim the buffers if they were already
171 consumed by the device:
173 .. kernel-doc:: drivers/virtio/virtio_ring.c
174 :identifiers: virtqueue_get_buf_ctx
176 The virtqueue callbacks can be disabled and re-enabled using the
177 virtqueue_disable_cb() and the family of virtqueue_enable_cb() functions
178 respectively. See drivers/virtio/virtio_ring.c for more details:
180 .. kernel-doc:: drivers/virtio/virtio_ring.c
181 :identifiers: virtqueue_disable_cb
183 .. kernel-doc:: drivers/virtio/virtio_ring.c
184 :identifiers: virtqueue_enable_cb
186 But note that some spurious callbacks can still be triggered under
187 certain scenarios. The way to disable callbacks reliably is to reset the
188 device or the virtqueue (virtio_reset_device()).
194 _`[1]` Virtio Spec v1.2:
195 https://docs.oasis-open.org/virtio/virtio/v1.2/virtio-v1.2.html
197 Check for later versions of the spec as well.