Basic doc about Virtio on Linux and a short tutorial on Virtio drivers.
Signed-off-by: Ricardo Cañuelo <ricardo.canuelo@xxxxxxxxxxxxx>
Reviewed-by: Cornelia Huck <cohuck@xxxxxxxxxx>
---
Documentation/driver-api/index.rst | 1 +
Documentation/driver-api/virtio/index.rst | 11 +
Documentation/driver-api/virtio/virtio.rst | 144 +++++++++++++
.../virtio/writing_virtio_drivers.rst | 197 ++++++++++++++++++
MAINTAINERS | 1 +
5 files changed, 354 insertions(+)
create mode 100644 Documentation/driver-api/virtio/index.rst
create mode 100644 Documentation/driver-api/virtio/virtio.rst
create mode 100644 Documentation/driver-api/virtio/writing_virtio_drivers.rst
diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
index d3a58f77328e..30a3de452b1d 100644
--- a/Documentation/driver-api/index.rst
+++ b/Documentation/driver-api/index.rst
@@ -106,6 +106,7 @@ available subsections can be seen below.
vfio-mediated-device
vfio
vfio-pci-device-specific-driver-acceptance
+ virtio/index
xilinx/index
xillybus
zorro
diff --git a/Documentation/driver-api/virtio/index.rst b/Documentation/driver-api/virtio/index.rst
new file mode 100644
index 000000000000..528b14b291e3
--- /dev/null
+++ b/Documentation/driver-api/virtio/index.rst
@@ -0,0 +1,11 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+======
+Virtio
+======
+
+.. toctree::
+ :maxdepth: 1
+
+ virtio
+ writing_virtio_drivers
diff --git a/Documentation/driver-api/virtio/virtio.rst b/Documentation/driver-api/virtio/virtio.rst
new file mode 100644
index 000000000000..921758fc3b23
--- /dev/null
+++ b/Documentation/driver-api/virtio/virtio.rst
@@ -0,0 +1,144 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _virtio:
+
+===============
+Virtio on Linux
+===============
+
+Introduction
+============
+
+Virtio is an open standard that defines a protocol for communication
+between drivers and devices of different types, see Chapter 5 ("Device
+Types") of the virtio spec `[1]`_. Originally developed as a standard
+for paravirtualized devices implemented by a hypervisor, it can be used
+to interface any compliant device (real or emulated) with a driver.
+
+For illustrative purposes, this document will focus on the common case
+of a Linux kernel running in a virtual machine and using paravirtualized
+devices provided by the hypervisor, which exposes them as virtio devices
+via standard mechanisms such as PCI.
+
+
+Device - Driver communication: virtqueues
+=========================================
+
+Although the virtio devices are really an abstraction layer in the
+hypervisor, they're exposed to the guest as if they are physical devices
+using a specific transport method -- PCI, MMIO or CCW -- that is
+orthogonal to the device itself. The virtio spec defines these transport
+methods in detail, including device discovery, capabilities and
+interrupt handling.
+
+The communication between the driver in the guest OS and the device in
+the hypervisor is done through shared memory (that's what makes virtio
+devices so efficient) using specialized data structures called
+virtqueues, which are actually ring buffers [#f1]_ of buffer descriptors
+similar to the ones used in a network device:
+
+.. kernel-doc:: include/uapi/linux/virtio_ring.h
+ :identifiers: struct vring_desc
+
+All the buffers the descriptors point to are allocated by the guest and
+used by the host either for reading or for writing but not for both.
+
+Refer to Chapter 2.5 ("Virtqueues") of the virtio spec `[1]`_ for the
+reference definitions of virtqueues and to `[2]`_ for an illustrated
+overview of how the host device and the guest driver communicate.
+
+The :c:type:`vring_virtqueue` struct models a virtqueue, including the
+ring buffers and management data. Embedded in this struct is the
+:c:type:`virtqueue` struct, which is the data structure that's
+ultimately used by virtio drivers:
+
+.. kernel-doc:: include/linux/virtio.h
+ :identifiers: struct virtqueue
+
+The callback function pointed by this struct is triggered when the
+device has consumed the buffers provided by the driver. More
+specifically, the trigger will be an interrupt issued by the hypervisor
+(see vring_interrupt()). Interrupt request handlers are registered for
+a virtqueue during the virtqueue setup process (transport-specific).
+
+.. kernel-doc:: drivers/virtio/virtio_ring.c
+ :identifiers: vring_interrupt
+
+
+Device discovery and probing
+============================
+
+In the kernel, the virtio core contains the virtio bus driver and
+transport-specific drivers like `virtio-pci` and `virtio-mmio`. Then
+there are individual virtio drivers for specific device types that are
+registered to the virtio bus driver.
+
+How a virtio device is found and configured by the kernel depends on how
+the hypervisor defines it. Taking the `QEMU virtio-console
+<https://gitlab.com/qemu-project/qemu/-/blob/master/hw/char/virtio-console.c>`__
+device as an example. When using PCI as a transport method, the device
+will present itself on the PCI bus with vendor 0x1af4 (Red Hat, Inc.)
+and device id 0x1003 (virtio console), as defined in the spec, so the
+kernel will detect it as it would do with any other PCI device.
+
+During the PCI enumeration process, if a device is found to match the
+virtio-pci driver (according to the virtio-pci device table, any PCI
+device with vendor id = 0x1af4)::
+
+ /* Qumranet donated their vendor ID for devices 0x1000 thru 0x10FF. */
+ static const struct pci_device_id virtio_pci_id_table[] = {
+ { PCI_DEVICE(PCI_VENDOR_ID_REDHAT_QUMRANET, PCI_ANY_ID) },
+ { 0 }
+ };
+
+then the virtio-pci driver is probed and, if the probing goes well, the
+device is registered to the virtio bus::
+
+ static int virtio_pci_probe(struct pci_dev *pci_dev,
+ const struct pci_device_id *id)
+ {
+ ...
+
+ if (force_legacy) {
+ rc = virtio_pci_legacy_probe(vp_dev);
+ /* Also try modern mode if we can't map BAR0 (no IO space). */
+ if (rc == -ENODEV || rc == -ENOMEM)
+ rc = virtio_pci_modern_probe(vp_dev);
+ if (rc)
+ goto err_probe;
+ } else {
+ rc = virtio_pci_modern_probe(vp_dev);
+ if (rc == -ENODEV)
+ rc = virtio_pci_legacy_probe(vp_dev);
+ if (rc)
+ goto err_probe;
+ }
+
+ ...
+
+ rc = register_virtio_device(&vp_dev->vdev);
+
+When the device is registered to the virtio bus the kernel will look
+for a driver in the bus that can handle the device and call that
+driver's ``probe`` method.
+
+It's at this stage that the virtqueues will be allocated and configured
+by calling the appropriate ``virtio_find`` helper function, such as
+virtio_find_single_vq() or virtio_find_vqs(), which will end up
+calling a transport-specific ``find_vqs`` method.
+
+
+References
+==========
+
+_`[1]` Virtio Spec v1.2:
+https://docs.oasis-open.org/virtio/virtio/v1.2/virtio-v1.2.html
+
+Check for later versions of the spec as well.
+
+_`[2]` Virtqueues and virtio ring: How the data travels
+https://www.redhat.com/en/blog/virtqueues-and-virtio-ring-how-data-travels
+
+.. rubric:: Footnotes
+
+.. [#f1] that's why they may be also referred to as virtrings.
diff --git a/Documentation/driver-api/virtio/writing_virtio_drivers.rst b/Documentation/driver-api/virtio/writing_virtio_drivers.rst
new file mode 100644
index 000000000000..e14c58796d25
--- /dev/null
+++ b/Documentation/driver-api/virtio/writing_virtio_drivers.rst
@@ -0,0 +1,197 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _writing_virtio_drivers:
+
+======================
+Writing Virtio Drivers
+======================
+
+Introduction
+============
+
+This document serves as a basic guideline for driver programmers that
+need to hack a new virtio driver or understand the essentials of the
+existing ones. See :ref:`Virtio on Linux <virtio>` for a general
+overview of virtio.
+
+
+Driver boilerplate
+==================
+
+As a bare minimum, a virtio driver needs to register in the virtio bus
+and configure the virtqueues for the device according to its spec, the
+configuration of the virtqueues in the driver side must match the
+virtqueue definitions in the device. A basic driver skeleton could look
+like this::
+
+ #include <linux/virtio.h>
+ #include <linux/virtio_ids.h>
+ #include <linux/virtio_config.h>
+ #include <linux/module.h>
+
+ /* device private data (one per device) */
+ struct virtio_dummy_dev {
+ struct virtqueue *vq;
+ };
+
+ static void virtio_dummy_recv_cb(struct virtqueue *vq)
+ {
+ struct virtio_dummy_dev *dev = vq->vdev->priv;
+ char *buf;
+ unsigned int len;
+
+ while ((buf = virtqueue_get_buf(dev->vq, &len)) != NULL) {
+ /* process the received data */
+ }
+ }
+
+ static int virtio_dummy_probe(struct virtio_device *vdev)
+ {
+ struct virtio_dummy_dev *dev = NULL;
+
+ /* initialize device data */
+ dev = kzalloc(sizeof(struct virtio_dummy_dev), GFP_KERNEL);
+ if (!dev)
+ return -ENOMEM;
+
+ /* the device has a single virtqueue */
+ dev->vq = virtio_find_single_vq(vdev, virtio_dummy_recv_cb, "input");
+ if (IS_ERR(dev->vq)) {
+ kfree(dev);
+ return PTR_ERR(dev->vq);
+
+ }
+ vdev->priv = dev;
+
+ /* from this point on, the device can notify and get callbacks */
+ virtio_device_ready(vdev);
+
+ return 0;
+ }
+
+ static void virtio_dummy_remove(struct virtio_device *vdev)
+ {
+ struct virtio_dummy_dev *dev = vdev->priv;
+
+ /*
+ * disable vq interrupts: equivalent to
+ * vdev->config->reset(vdev)
+ */
+ virtio_reset_device(vdev);
+
+ /* detach unused buffers */
+ while ((buf = virtqueue_detach_unused_buf(dev->vq)) != NULL) {
+ kfree(buf);
+ }
+
+ /* remove virtqueues */
+ vdev->config->del_vqs(vdev);
+
+ kfree(dev);
+ }
+
+ static const struct virtio_device_id id_table[] = {
+ { VIRTIO_ID_DUMMY, VIRTIO_DEV_ANY_ID },
+ { 0 },
+ };
+
+ static struct virtio_driver virtio_dummy_driver = {
+ .driver.name = KBUILD_MODNAME,
+ .driver.owner = THIS_MODULE,
+ .id_table = id_table,
+ .probe = virtio_dummy_probe,
+ .remove = virtio_dummy_remove,
+ };
+
+ module_virtio_driver(virtio_dummy_driver);
+ MODULE_DEVICE_TABLE(virtio, id_table);
+ MODULE_DESCRIPTION("Dummy virtio driver");
+ MODULE_LICENSE("GPL");
+
+The device id ``VIRTIO_ID_DUMMY`` here is a placeholder, virtio drivers
+should be added only for devices that are defined in the spec, see
+include/uapi/linux/virtio_ids.h. Device ids need to be at least reserved
+in the virtio spec before being added to that file.
+
+If your driver doesn't have to do anything special in its ``init`` and
+``exit`` methods, you can use the module_virtio_driver() helper to
+reduce the amount of boilerplate code.
+
+The ``probe`` method does the minimum driver setup in this case
+(memory allocation for the device data) and initializes the
+virtqueue. virtio_device_ready() is used to enable the virtqueue and to
+notify the device that the driver is ready to manage the device
+("DRIVER_OK"). The virtqueues are anyway enabled automatically by the
+core after ``probe`` returns.
+
+.. kernel-doc:: include/linux/virtio_config.h
+ :identifiers: virtio_device_ready
+
+In any case, the virtqueues need to be enabled before adding buffers to
+them.
+
+Sending and receiving data
+==========================
+
+The virtio_dummy_recv_cb() callback in the code above will be triggered
+when the device notifies the driver after it finishes processing a
+descriptor or descriptor chain, either for reading or writing. However,
+that's only the second half of the virtio device-driver communication
+process, as the communication is always started by the driver regardless
+of the direction of the data transfer.
+
+To configure a buffer transfer from the driver to the device, first you
+have to add the buffers -- packed as `scatterlists` -- to the
+appropriate virtqueue using any of the virtqueue_add_inbuf(),
+virtqueue_add_outbuf() or virtqueue_add_sgs(), depending on whether you
+need to add one input `scatterlist` (for the device to fill in), one
+output `scatterlist` (for the device to consume) or multiple
+`scatterlists`, respectively. Then, once the virtqueue is set up, a call
+to virtqueue_kick() sends a notification that will be serviced by the
+hypervisor that implements the device::
+
+ struct scatterlist sg[1];
+ sg_init_one(sg, buffer, BUFLEN);
+ virtqueue_add_inbuf(dev->vq, sg, 1, buffer, GFP_ATOMIC);
+ virtqueue_kick(dev->vq);
+
+.. kernel-doc:: drivers/virtio/virtio_ring.c
+ :identifiers: virtqueue_add_inbuf
+
+.. kernel-doc:: drivers/virtio/virtio_ring.c
+ :identifiers: virtqueue_add_outbuf
+
+.. kernel-doc:: drivers/virtio/virtio_ring.c
+ :identifiers: virtqueue_add_sgs
+
+Then, after the device has read or written the buffers prepared by the
+driver and notifies it back, the driver can call virtqueue_get_buf() to
+read the data produced by the device (if the virtqueue was set up with
+input buffers) or simply to reclaim the buffers if they were already
+consumed by the device:
+
+.. kernel-doc:: drivers/virtio/virtio_ring.c
+ :identifiers: virtqueue_get_buf_ctx
+
+The virtqueue callbacks can be disabled and re-enabled using the
+virtqueue_disable_cb() and the family of virtqueue_enable_cb() functions
+respectively. See drivers/virtio/virtio_ring.c for more details:
+
+.. kernel-doc:: drivers/virtio/virtio_ring.c
+ :identifiers: virtqueue_disable_cb
+
+.. kernel-doc:: drivers/virtio/virtio_ring.c
+ :identifiers: virtqueue_enable_cb
+
+But note that some spurious callbacks can still be triggered under
+certain scenarios. The way to disable callbacks reliably is to reset the
+device or the virtqueue (virtio_reset_device()).
+
+
+References
+==========
+
+_`[1]` Virtio Spec v1.2:
+https://docs.oasis-open.org/virtio/virtio/v1.2/virtio-v1.2.html
+
+Check for later versions of the spec as well.
diff --git a/MAINTAINERS b/MAINTAINERS
index 8a5012ba6ff9..ec59f1189d46 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -21476,6 +21476,7 @@ S: Maintained
F: Documentation/ABI/testing/sysfs-bus-vdpa
F: Documentation/ABI/testing/sysfs-class-vduse
F: Documentation/devicetree/bindings/virtio/
+F: Documentation/driver-api/virtio/
F: drivers/block/virtio_blk.c
F: drivers/crypto/virtio/
F: drivers/net/virtio_net.c
--
2.25.1
