Virtualization - Aug 2022 - [PATCH v2 2/2] docs: driver-api: virtio: virtio on Linux

Basic doc about Virtio on Linux and a short tutorial on Virtio drivers.

Signed-off-by: Ricardo Ca?uelo <ricardo.canuelo at collabora.com>
---
 Documentation/driver-api/index.rst            |   1 +
 Documentation/driver-api/virtio/index.rst     |  11 +
 Documentation/driver-api/virtio/virtio.rst    | 151 ++++++++++++++
 .../virtio/writing_virtio_drivers.rst         | 189 ++++++++++++++++++
 MAINTAINERS                                   |   1 +
 5 files changed, 353 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..4b73c705c94c
--- /dev/null
+++ b/Documentation/driver-api/virtio/virtio.rst
@@ -0,0 +1,151 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _virtio:
+
+==============+Virtio on Linux
+==============+
+Introduction
+===========+
+Virtio is an open standard interface for virtual machines to access
+paravirtualized devices, ie. devices that aren't emulated by a
+hypervisor but rather real host devices that are exposed by the
+hypervisor to the guest to achieve native performance. In other words,
+it provides a communication mechanism for a guest OS to use devices on
+the host machine. Additionally, some devices also implement the virtio
+interface in hardware.
+
+For paravirtualized devices, the concrete hardware details of the real
+host devices are abstracted in the hypervisor, which provides a set of
+simplified virtual devices that implement the virtio protocol. These
+devices are defined in Chapter 5 ("Device Types") of the virtio spec
[1]
+and they're the devices that the guest OS will ultimately handle. So, in
+that regard, the guest OS knows it's running in a virtual environment
+and that it needs to use the appropriate virtio drivers to handle the
+devices instead of the regular device drivers it'd use in a native or
+purely virtual environment (with emulated devices).
+
+
+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 in the PCI bus with vendor 0x1af4 (RedHat, 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 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..139c785a38ef
--- /dev/null
+++ b/Documentation/driver-api/virtio/writing_virtio_drivers.rst
@@ -0,0 +1,189 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _writing_virtio_drivers:
+
+=====================+Writing Virtio Drivers
+=====================+
+Introduction
+===========+
+Chapter 5 ("Device Types") in the virtio specification [1] defines
all
+the supported virtio device types. Since these devices are, by
+definition, meant as abstractions for a wide variety of real hardware,
+the addition of new virtio drivers is not expected to be very
+frequent. Still, 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 should 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;
+
+		buf = virtqueue_get_buf(dev->vq, &len);
+		/* spurious callback? */
+		if (!buf)
+			return;
+
+		/* 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;
+
+		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);
+
+		/* 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 defined only for devices that are defined in the
+spec. See include/uapi/linux/virtio_ids.h.
+
+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. The virtqueues are automatically enabled after ``probe``
+returns, sending the appropriate "DRIVER_OK" status signal to the
+device. If the virtqueues need to be enabled before ``probe`` ends, they
+can be manually enabled by calling virtio_device_ready():
+
+.. kernel-doc:: include/linux/virtio_config.h
+    :identifiers: virtio_device_ready
+
+
+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
+
+
+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 779f599f9abf..6ecdddb89da4 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -21488,6 +21488,7 @@ 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

On Thu, Aug 04 2022, Ricardo Ca?uelo <ricardo.canuelo at collabora.com>
wrote:
> Basic doc about Virtio on Linux and a short tutorial on Virtio drivers.
>
> Signed-off-by: Ricardo Ca?uelo <ricardo.canuelo at collabora.com>
> ---
>  Documentation/driver-api/index.rst            |   1 +
>  Documentation/driver-api/virtio/index.rst     |  11 +
>  Documentation/driver-api/virtio/virtio.rst    | 151 ++++++++++++++
>  .../virtio/writing_virtio_drivers.rst         | 189 ++++++++++++++++++
>  MAINTAINERS                                   |   1 +
>  5 files changed, 353 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/virtio/virtio.rst
b/Documentation/driver-api/virtio/virtio.rst
> new file mode 100644
> index 000000000000..4b73c705c94c
> --- /dev/null
> +++ b/Documentation/driver-api/virtio/virtio.rst
> @@ -0,0 +1,151 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +.. _virtio:
> +
> +==============> +Virtio on Linux
> +==============> +
> +Introduction
> +===========> +
> +Virtio is an open standard interface for virtual machines to access
> +paravirtualized devices, ie. devices that aren't emulated by a
> +hypervisor but rather real host devices that are exposed by the
> +hypervisor to the guest to achieve native performance. In other words,
> +it provides a communication mechanism for a guest OS to use devices on
> +the host machine. Additionally, some devices also implement the virtio
> +interface in hardware.
> +
> +For paravirtualized devices, the concrete hardware details of the real
> +host devices are abstracted in the hypervisor, which provides a set of
> +simplified virtual devices that implement the virtio protocol. These
> +devices are defined in Chapter 5 ("Device Types") of the virtio
spec [1]
> +and they're the devices that the guest OS will ultimately handle. So,
in
> +that regard, the guest OS knows it's running in a virtual environment
> +and that it needs to use the appropriate virtio drivers to handle the
> +devices instead of the regular device drivers it'd use in a native or
> +purely virtual environment (with emulated devices).
Hm... so I'm not quite happy with those two paragraphs, but I'm not sure
how to make it better.

- While the origins of virtio are hypervisor implementations, the
  standard is describing the mechanisms of device <-> driver
  communication, regardless whether the device is a hardware entity or
  something emulated by a hypervisor.
- I'm not quite sure what you are referring to with "real host
  devices". We can have e.g. a real disk that is handed to a guest via
  virtio-blk as a whole, or we can have some kind of file that is
  exposed via virtio-blk. Other device types can also be completely
  emulated.
- The OS picks the driver depending on what device is discovers; virtio
  device drivers are not any different from other device drivers in that
  regard.

So I think the key pieces of virtio are the following:

- it is an open standard
- it describes a protocol, which can be used to implement a lot of
  different device types
- those devices are exposed to the operating system via standard
  mechanisms such as PCI
- virtio devices can be implemented in various ways, such as in
  hypervisors (more common) or as a real hardware device

For the remainder of this document, it is probably fine to focus on the
hypervisor use case.

(...)
> +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 in the PCI bus with vendor 0x1af4 (RedHat, Inc.) and
s/in/on/ ?
s/RedHat/Red Hat/ :)
> +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.
(...)
> 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..139c785a38ef
> --- /dev/null
> +++ b/Documentation/driver-api/virtio/writing_virtio_drivers.rst
> @@ -0,0 +1,189 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +.. _writing_virtio_drivers:
> +
> +=====================> +Writing Virtio Drivers
> +=====================> +
> +Introduction
> +===========> +
> +Chapter 5 ("Device Types") in the virtio specification [1]
defines all
> +the supported virtio device types. Since these devices are, by
> +definition, meant as abstractions for a wide variety of real hardware,
See my comments above, virtio devices can also be fully emulated or real
hardware devices.
> +the addition of new virtio drivers is not expected to be very
> +frequent. Still, this document serves as a basic guideline for driver
I think we should not make any statement regarding frequency of new
additions; sometimes, there's a flurry of activity, sometimes, there's
nothing for ages :)
> +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 should register in the virtio bus and
s/should/needs to/ ?
> +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::
(...)
> +The device id ``VIRTIO_ID_DUMMY`` here is a placeholder, virtio
> +drivers should be defined only for devices that are defined in the
s/defined/added/ ?
> +spec. See include/uapi/linux/virtio_ids.h.
Maybe "Device ids need to be at least reserved in the virtio spec before
being added to that file." ?