.. _hld-virtio-devices:
.. _virtio-hld:

Virtio Devices High-Level Design
################################

The ACRN hypervisor follows the `Virtual I/O Device (virtio)
specification
<http://docs.oasis-open.org/virtio/virtio/v1.0/virtio-v1.0.html>`_ to
realize I/O virtualization for many performance-critical devices
supported in the ACRN project. Adopting the virtio specification lets us
reuse many frontend virtio drivers already available in a Linux-based
User VM, drastically reducing potential development effort for frontend
virtio drivers.  To further reduce the development effort of backend
virtio drivers, the hypervisor  provides the virtio backend service
(VBS) APIs, that make  it very straightforward to implement a virtio
device in the hypervisor.

The virtio APIs can be divided into 3 groups: DM APIs, virtio backend
service (VBS) APIs, and virtqueue (VQ) APIs, as shown in
:numref:`be-interface`.

.. figure:: images/virtio-hld-image0.png
   :width: 900px
   :align: center
   :name: be-interface

   ACRN Virtio Backend Service Interface

-  **DM APIs** are exported by the DM, and are mainly used during the
   device initialization phase and runtime. The DM APIs also include
   PCIe emulation APIs because each virtio device is a PCIe device in
   the Service VM and User VM.
-  **VBS APIs** are mainly exported by the VBS and related modules.
   Generally they are callbacks to be
   registered into the DM.
-  **VQ APIs** are used by a virtio backend device to access and parse
   information from the shared memory between the frontend and backend
   device drivers.

Virtio framework is the para-virtualization specification that ACRN
follows to implement I/O virtualization of performance-critical
devices such as audio, eAVB/TSN, IPU, and CSMU devices. This section gives
an overview about virtio history, motivation, and advantages, and then
highlights virtio key concepts. Second, this section will describe
ACRN's virtio architectures and elaborate on ACRN virtio APIs. Finally
this section will introduce all the virtio devices supported
by ACRN.

Virtio Introduction
*******************

Virtio is an abstraction layer over devices in a para-virtualized
hypervisor. Virtio was developed by Rusty Russell when he worked at IBM
research to support his lguest hypervisor in 2007, and it quickly became
the de facto standard for KVM's para-virtualized I/O devices.

Virtio is very popular for virtual I/O devices because it provides a
straightforward, efficient, standard, and extensible mechanism, and
eliminates the need for boutique, per-environment, or per-OS mechanisms.
For example, rather than having a variety of device emulation
mechanisms, virtio provides a common frontend driver framework that
standardizes device interfaces, and increases code reuse across
different virtualization platforms.

Given the advantages of virtio, ACRN also follows the virtio
specification.

Key Concepts
************

To better understand virtio, especially its usage in ACRN, we'll
highlight several key virtio concepts important to ACRN:


Frontend virtio driver (FE)
  Virtio adopts a frontend-backend architecture that enables a simple but
  flexible framework for both frontend and backend virtio drivers. The FE
  driver merely needs to offer services that configure the interface, pass messages,
  produce requests, and kick the backend virtio driver. As a result, the FE
  driver is easy to implement and the performance overhead of emulating
  a device is eliminated.

Backend virtio driver (BE)
  Similar to the FE driver, the BE driver, running either in userland or
  kernel-land of the host OS, consumes requests from the FE driver and sends them
  to the host native device driver. Once the requests are done by the host
  native device driver, the BE driver notifies the FE driver that the
  request is complete.

  Note: To distinguish the BE driver from the host native device driver, the
  host native device driver is called "native driver" in this document.

Straightforward: virtio devices as standard devices on existing buses
  Instead of creating new device buses from scratch, virtio devices are
  built on existing buses. This gives a straightforward way for both FE
  and BE drivers to interact with each other. For example, the FE driver could
  read/write registers of the device, and the virtual device could
  interrupt the FE driver, on behalf of the BE driver, in case something of
  interest is happening.

  The virtio supports PCI/PCIe bus and MMIO bus. In ACRN, only
  PCI/PCIe bus is supported, and all the virtio devices share the same
  vendor ID 0x1AF4.

  Note: For MMIO, the "bus" is an overstatement since
  basically it is a few descriptors describing the devices.

Efficient: batching operation is encouraged
  Batching operation and deferred notification are important to achieve
  high-performance I/O, since notification between the FE driver and BE driver
  usually involves an expensive exit of the guest. Therefore batching
  operating and notification suppression are highly encouraged if
  possible. This will give an efficient implementation for
  performance-critical devices.

Standard: virtqueue
  All virtio devices share a standard ring buffer and descriptor
  mechanism, called a virtqueue, shown in :numref:`virtqueue`. A virtqueue is a
  queue of scatter-gather buffers. There are three important methods on
  virtqueues:

  - **add_buf** is for adding a request/response buffer in a virtqueue,
  - **get_buf** is for getting a response/request in a virtqueue, and
  - **kick** is for notifying the other side for a virtqueue to consume buffers.

  The virtqueues are created in guest physical memory by the FE drivers.
  BE drivers only need to parse the virtqueue structures to obtain
  the requests and process them. The virtqueue organization is
  specific to the Guest OS. In the Linux implementation of virtio, the
  virtqueue is implemented as a ring buffer structure called `vring``.

  In ACRN, the virtqueue APIs can be leveraged directly so that users
  don't need to worry about the details of the virtqueue. (Refer to guest
  OS for more details about the virtqueue implementation.)

.. figure:: images/virtio-hld-image2.png
   :width: 900px
   :align: center
   :name: virtqueue

   Virtqueue

Extensible: feature bits
  A simple extensible feature negotiation mechanism exists for each
  virtual device and its driver. Each virtual device could claim its
  device specific features while the corresponding driver could respond to
  the device with the subset of features the driver understands. The
  feature mechanism enables forward and backward compatibility for the
  virtual device and driver.

Virtio Device Modes
  The virtio specification defines three modes of virtio devices:
  a legacy mode device, a transitional mode device, and a modern mode
  device. A legacy mode device is compliant to virtio specification
  version 0.95, a transitional mode device is compliant to both
  0.95 and 1.0 spec versions, and a modern mode
  device is only compatible to the version 1.0 specification.

  In ACRN, all the virtio devices are transitional devices, meaning that
  they should be compatible with both the 0.95 and 1.0 versions of the virtio
  specification.

Virtio Device Discovery
  Virtio devices are commonly implemented as PCI/PCIe devices. A
  virtio device using virtio over a PCI/PCIe bus must expose an interface to
  the Guest OS that meets the PCI/PCIe specifications.

  Conventionally, any PCI device with Vendor ID 0x1AF4,
  PCI_VENDOR_ID_REDHAT_QUMRANET, and Device ID 0x1000 through 0x107F
  inclusive is a virtio device. Among the Device IDs, the
  legacy/transitional mode virtio devices occupy the first 64 IDs ranging
  from 0x1000 to 0x103F, while the range 0x1040-0x107F belongs to
  virtio modern devices. In addition, the Subsystem Vendor ID should
  reflect the PCI/PCIe vendor ID of the environment, and the Subsystem
  Device ID indicates which virtio device is supported by the device.

Virtio Frameworks
*****************

This section describes the overall architecture of virtio, and
introduces the ACRN-specific implementations of the virtio framework.

Architecture
============

Virtio adopts a frontend-backend
architecture, as shown in :numref:`virtio-arch`. Basically the FE driver and BE
driver
communicate with each other through shared memory, via the
virtqueues. The FE driver talks to the BE driver in the same way it
would talk to a real PCIe device. The BE driver handles requests
from the FE driver, and notifies the FE driver if the request has been
processed.

.. figure:: images/virtio-hld-image1.png
   :width: 900px
   :align: center
   :name: virtio-arch

   Virtio Architecture

In addition to virtio's frontend-backend architecture, both FE and BE
drivers follow a layered architecture, as shown in
:numref:`virtio-fe-be`. Each
side has three layers: transports, core models, and device types.
All virtio devices share the same virtio infrastructure, including
virtqueues, feature mechanisms, configuration space, and buses.

.. figure:: images/virtio-hld-image4.png
   :width: 900px
   :align: center
   :name: virtio-fe-be

   Virtio Frontend/Backend Layered Architecture

Userland Virtio Framework
==========================

The architecture of ACRN userland virtio framework (VBS-U) is shown in
:numref:`virtio-userland`.

The FE driver talks to the BE driver as if it were talking with a PCIe
device. This means for "control plane", the FE driver could poke device
registers through PIO or MMIO, and the device will interrupt the FE
driver when something happens. For "data plane", the communication
between the FE driver and BE driver is through shared memory, in the form of
virtqueues.

On the Service VM side where the BE driver is located, there are several
key components in ACRN, including Device Model (DM), Hypervisor
service module (HSM), VBS-U, and user-level vring service API helpers.

DM bridges the FE driver and BE driver since each VBS-U module emulates
a PCIe virtio device. HSM bridges DM and the hypervisor by providing
remote memory map APIs and notification APIs. VBS-U accesses the
virtqueue through the user-level vring service API helpers.

.. figure:: images/virtio-hld-image3.png
   :width: 900px
   :align: center
   :name: virtio-userland

   ACRN Userland Virtio Framework

Kernel-Land Virtio Framework
============================

ACRN supports one kernel-land virtio frameworks:

* Vhost, compatible with Linux Vhost

Vhost Framework
---------------

Vhost is a common solution upstreamed in the Linux kernel,
with several kernel mediators based on it.

Architecture
~~~~~~~~~~~~

Vhost/virtio is a semi-virtualized device abstraction interface
specification that has been widely applied in various virtualization
solutions. Vhost is a specific kind of virtio where the data plane is
put into host kernel space to reduce the context switch while processing
the IO request. It is usually called "virtio" when used as a frontend
driver in a guest operating system or "vhost" when used as a backend
driver in a host. Compared with a pure virtio solution on a host, vhost
uses the same frontend driver as the virtio solution and can achieve better
performance. :numref:`vhost-arch` shows the vhost architecture on ACRN.

.. figure:: images/virtio-hld-image71.png
   :align: center
   :name: vhost-arch

   Vhost Architecture on ACRN

Compared with a userspace virtio solution, vhost decomposes data plane
from user space to kernel space. The vhost general data plane workflow
can be described as:

1. The vhost proxy creates two eventfds per virtqueue, one is for kick
   (an ioeventfd), the other is for call (an irqfd).
2. The vhost proxy registers the two eventfds to HSM through HSM character
   device:

   a) Ioeventfd is bound with a PIO/MMIO range. If it is a PIO, it is
      registered with ``(fd, port, len, value)``. If it is an MMIO, it is
      registered with ``(fd, addr, len)``.
   b) Irqfd is registered with MSI vector.

3. The vhost proxy sets the two fds to vhost kernel through ioctl of vhost
   device.
4. The vhost starts polling the kick fd and wakes up when the guest kicks a
   virtqueue, which results in an event_signal on the kick fd by the HSM
   ioeventfd.
5. The vhost device in the kernel signals on the irqfd to notify the guest.

Ioeventfd Implementation
~~~~~~~~~~~~~~~~~~~~~~~~

Ioeventfd module is implemented in HSM, and can enhance a registered
eventfd to listen to IO requests (PIO/MMIO) from the HSM ioreq module and
signal the eventfd when needed. :numref:`ioeventfd-workflow`  shows the
general workflow of ioeventfd.

.. figure:: images/virtio-hld-image58.png
   :align: center
   :name: ioeventfd-workflow

   Ioeventfd General Workflow

The workflow can be summarized as:

1. The vhost device initializes. The vhost proxy creates two eventfds for
   ioeventfd and irqfd.
2. The vhost proxy passes the ioeventfd to the vhost kernel driver.
3. The vhost proxy passes the ioeventfd to the HSM driver.
4. The User VM FE driver triggers an ioreq, which is forwarded through the
   hypervisor to the Service VM.
5. The HSM driver dispatches the ioreq to the related HSM client.
6. The ioeventfd HSM client traverses the io_range list and finds the
   corresponding eventfd.
7. The ioeventfd HSM client triggers the signal to the related eventfd.

Irqfd Implementation
~~~~~~~~~~~~~~~~~~~~

The irqfd module is implemented in HSM, and can enhance a registered
eventfd to inject an interrupt to a guest OS when the eventfd gets
signaled. :numref:`irqfd-workflow` shows the general flow for irqfd.

.. figure:: images/virtio-hld-image60.png
   :align: center
   :name: irqfd-workflow

   Irqfd General Flow

The workflow can be summarized as:

1. The vhost device initializes. The vhost proxy creates two eventfds for
   ioeventfd and irqfd.
2. The vhost proxy passes the irqfd to the vhost kernel driver.
3. The vhost proxy passes the irqfd to the HSM driver.
4. The vhost device driver triggers an IRQ eventfd signal once the related
   native transfer is completed.
5. The irqfd related logic traverses the irqfd list to retrieve related irq
   information.
6. The irqfd related logic injects an interrupt through the HSM interrupt API.
7. The interrupt is delivered to the User VM FE driver through the hypervisor.

.. _virtio-APIs:

Virtio APIs
***********

This section provides details on the ACRN virtio APIs. As outlined previously,
the ACRN virtio APIs can be divided into three groups: DM_APIs,
VBS_APIs, and VQ_APIs. The following sections will elaborate on
these APIs.

VBS-U Key Data Structures
=========================

The key data structures for VBS-U are listed as follows, and their
relationships are shown in :numref:`VBS-U-data`.

``struct pci_virtio_blk``
  An example virtio device, such as virtio-blk.
``struct virtio_common``
  A common component to any virtio device.
``struct virtio_ops``
  Virtio specific operation functions for this type of virtio device.
``struct pci_vdev``
  Instance of a virtual PCIe device, and any virtio
  device is a virtual PCIe device.
``struct pci_vdev_ops``
  PCIe device's operation functions for this type
  of device.
``struct vqueue_info``
  Instance of a virtqueue.

.. figure:: images/virtio-hld-image5.png
   :width: 900px
   :align: center
   :name: VBS-U-data

   VBS-U Key Data Structures

Each virtio device is a PCIe device. In addition, each virtio device
could have none or multiple virtqueues, depending on the device type.
The ``struct virtio_common`` is a key data structure to be manipulated by
DM, and DM finds other key data structures through it. The ``struct
virtio_ops`` abstracts a series of virtio callbacks to be provided by the
device owner.

VHOST Key Data Structures
=========================

The key data structures for vhost are listed as follows.

.. doxygenstruct:: vhost_dev
   :project: Project ACRN

.. doxygenstruct:: vhost_vq
   :project: Project ACRN

DM APIs
=======

The DM APIs are exported by DM, and they should be used when implementing
BE device drivers on ACRN.

.. doxygenfunction:: paddr_guest2host
   :project: Project ACRN

.. doxygenfunction:: pci_set_cfgdata8
   :project: Project ACRN

.. doxygenfunction:: pci_set_cfgdata16
   :project: Project ACRN

.. doxygenfunction:: pci_set_cfgdata32
   :project: Project ACRN

.. doxygenfunction:: pci_get_cfgdata8
   :project: Project ACRN

.. doxygenfunction:: pci_get_cfgdata16
   :project: Project ACRN

.. doxygenfunction:: pci_get_cfgdata32
   :project: Project ACRN

.. doxygenfunction:: pci_lintr_assert
   :project: Project ACRN

.. doxygenfunction:: pci_lintr_deassert
   :project: Project ACRN

.. doxygenfunction:: pci_generate_msi
   :project: Project ACRN

.. doxygenfunction:: pci_generate_msix
   :project: Project ACRN

VBS APIs
========

The VBS APIs are exported by VBS related modules, including VBS, DM, and
Service VM kernel modules.

VBS-U APIs
----------

These APIs provided by VBS-U are callbacks to be registered to DM, and
the virtio framework within DM will invoke them appropriately.

.. doxygenstruct:: virtio_ops
   :project: Project ACRN

.. doxygenfunction:: virtio_pci_read
   :project: Project ACRN

.. doxygenfunction:: virtio_pci_write
   :project: Project ACRN

.. doxygenfunction:: virtio_interrupt_init
   :project: Project ACRN

.. doxygenfunction:: virtio_linkup
   :project: Project ACRN

.. doxygenfunction:: virtio_reset_dev
   :project: Project ACRN

.. doxygenfunction:: virtio_set_io_bar
   :project: Project ACRN

.. doxygenfunction:: virtio_set_modern_bar
   :project: Project ACRN

.. doxygenfunction:: virtio_config_changed
   :project: Project ACRN

APIs Provided by DM
~~~~~~~~~~~~~~~~~~~

.. doxygenfunction:: vbs_kernel_reset
   :project: Project ACRN

.. doxygenfunction:: vbs_kernel_start
   :project: Project ACRN

.. doxygenfunction:: vbs_kernel_stop
   :project: Project ACRN

VHOST APIs
==========

APIs Provided by DM
-------------------

.. doxygenfunction:: vhost_dev_init
   :project: Project ACRN

.. doxygenfunction:: vhost_dev_deinit
   :project: Project ACRN

.. doxygenfunction:: vhost_dev_start
   :project: Project ACRN

.. doxygenfunction:: vhost_dev_stop
   :project: Project ACRN

Linux Vhost IOCTLs
------------------

``#define VHOST_GET_FEATURES      _IOR(VHOST_VIRTIO, 0x00, __u64)``
  This IOCTL is used to get the supported feature flags by vhost kernel driver.
``#define VHOST_SET_FEATURES      _IOW(VHOST_VIRTIO, 0x00, __u64)``
  This IOCTL is used to set the supported feature flags to vhost kernel driver.
``#define VHOST_SET_OWNER _IO(VHOST_VIRTIO, 0x01)``
  This IOCTL is used to set the current process as the exclusive owner of the
  vhost char device. It must be called before any other vhost commands.
``#define VHOST_RESET_OWNER _IO(VHOST_VIRTIO, 0x02)``
  This IOCTL is used to give up the ownership of the vhost char device.
``#define VHOST_SET_MEM_TABLE     _IOW(VHOST_VIRTIO, 0x03, struct vhost_memory)``
  This IOCTL is used to convey the guest OS memory layout to the vhost kernel driver.
``#define VHOST_SET_VRING_NUM _IOW(VHOST_VIRTIO, 0x10, struct vhost_vring_state)``
  This IOCTL is used to set the number of descriptors in the virtio ring. It
  cannot be modified while the virtio ring is running.
``#define VHOST_SET_VRING_ADDR _IOW(VHOST_VIRTIO, 0x11, struct vhost_vring_addr)``
  This IOCTL is used to set the address of the virtio ring.
``#define VHOST_SET_VRING_BASE _IOW(VHOST_VIRTIO, 0x12, struct vhost_vring_state)``
  This IOCTL is used to set the base value where virtqueue looks for available
  descriptors.
``#define VHOST_GET_VRING_BASE _IOWR(VHOST_VIRTIO, 0x12, struct vhost_vring_state)``
  This IOCTL is used to get the base value where virtqueue looks for available
  descriptors.
``#define VHOST_SET_VRING_KICK _IOW(VHOST_VIRTIO, 0x20, struct vhost_vring_file)``
  This IOCTL is used to set the eventfd on which vhost can poll for guest
  virtqueue kicks.
``#define VHOST_SET_VRING_CALL _IOW(VHOST_VIRTIO, 0x21, struct vhost_vring_file)``
  This IOCTL is used to set the eventfd that is used by vhost to inject
  virtual interrupts.

HSM Eventfd IOCTLs
------------------

.. doxygenstruct:: acrn_ioeventfd
   :project: Project ACRN

``#define IC_EVENT_IOEVENTFD              _IC_ID(IC_ID, IC_ID_EVENT_BASE + 0x00)``
  This IOCTL is used to register or unregister an ioeventfd with the appropriate
  address, length, and data value.

.. doxygenstruct:: acrn_irqfd
   :project: Project ACRN

``#define IC_EVENT_IRQFD                  _IC_ID(IC_ID, IC_ID_EVENT_BASE + 0x01)``
  This IOCTL is used to register or unregister an irqfd with the appropriate MSI
  information.

VQ APIs
=======

The virtqueue APIs, or VQ APIs, are used by a BE device driver to
access the virtqueues shared by the FE driver. The VQ APIs abstract the
details of virtqueues so that users don't need to worry about the data
structures within the virtqueues.

.. doxygenfunction:: vq_interrupt
   :project: Project ACRN

.. doxygenfunction:: vq_getchain
   :project: Project ACRN

.. doxygenfunction:: vq_retchain
   :project: Project ACRN

.. doxygenfunction:: vq_relchain
   :project: Project ACRN

.. doxygenfunction:: vq_endchains
   :project: Project ACRN

Below is an example showing a typical logic of how a BE driver handles
requests from a FE driver.

.. code-block:: c

   static void BE_callback(struct pci_virtio_xxx *pv, struct vqueue_info *vq ) {
      while (vq_has_descs(vq)) {
         vq_getchain(vq, &idx, &iov, 1, NULL);
                /* handle requests in iov */
                request_handle_proc();
                /* Release this chain and handle more */
                vq_relchain(vq, idx, len);
         }
      /* Generate interrupt if appropriate. 1 means ring empty \*/
      vq_endchains(vq, 1);
   }

Supported Virtio Devices
************************

All the BE virtio drivers are implemented using the
ACRN virtio APIs, and the FE drivers reuse the standard Linux FE
virtio drivers. For the devices with FE drivers available in the Linux
kernel, they should use standard virtio Vendor ID/Device ID and
Subsystem Vendor ID/Subsystem Device ID. For other devices within ACRN,
their temporary IDs are listed in the following table.

.. table:: Virtio Devices without Existing FE Drivers in Linux
   :align: center
   :name: virtio-device-table

   +--------------+-------------+-------------+-------------+-------------+
   | virtio       | Vendor ID   | Device ID   | Subvendor   | Subdevice   |
   | device       |             |             | ID          | ID          |
   +--------------+-------------+-------------+-------------+-------------+
   | RPMB         | 0x8086      | 0x8601      | 0x8086      | 0xFFFF      |
   +--------------+-------------+-------------+-------------+-------------+
   | HECI         | 0x8086      | 0x8602      | 0x8086      | 0xFFFE      |
   +--------------+-------------+-------------+-------------+-------------+
   | audio        | 0x8086      | 0x8603      | 0x8086      | 0xFFFD      |
   +--------------+-------------+-------------+-------------+-------------+
   | IPU          | 0x8086      | 0x8604      | 0x8086      | 0xFFFC      |
   +--------------+-------------+-------------+-------------+-------------+
   | TSN/AVB      | 0x8086      | 0x8605      | 0x8086      | 0xFFFB      |
   +--------------+-------------+-------------+-------------+-------------+
   | hyper_dmabuf | 0x8086      | 0x8606      | 0x8086      | 0xFFFA      |
   +--------------+-------------+-------------+-------------+-------------+
   | HDCP         | 0x8086      | 0x8607      | 0x8086      | 0xFFF9      |
   +--------------+-------------+-------------+-------------+-------------+
   | COREU        | 0x8086      | 0x8608      | 0x8086      | 0xFFF8      |
   +--------------+-------------+-------------+-------------+-------------+
   | I2C          | 0x8086      | 0x860a      | 0x8086      | 0xFFF6      |
   +--------------+-------------+-------------+-------------+-------------+
   | GPIO         | 0x8086      | 0x8609      | 0x8086      | 0xFFF7      |
   +--------------+-------------+-------------+-------------+-------------+

The following sections introduce the status of virtio devices
supported in ACRN.

.. toctree::
   :maxdepth: 1

   virtio-blk
   virtio-net
   virtio-input
   virtio-console
   virtio-rnd
   virtio-i2c
   virtio-gpio
