VirtIO
VirtIO is a standard for performant virtualized hardware for use within virtual machines, created and managed by the OASIS Open Virtual I/O Device TC. VirtIO supports a variety of devices, such as, but not limited to: block (e.g. hard drive), network card, GPU, etc. Each of these devices have their own specific configuration and features, but all use the same underlying VirtIO queue structure for relaying data and commands to & from the device.
The latest specification, 1.2, is available at https://docs.oasis-open.org/virtio/virtio/v1.2/csd01/virtio-v1.2-csd01.html.
VirtIO provides a common framework for devices to be identified and for communicating between the device implementation and the driver. Communication for is handled through so-called virtqueues.
Certain implementations extend the features available.
For example, here is the list of virtio-gpu capsets in Mesa, at src/gfxstream/guest/platform/include/virtgpu_gfxstream_protocol.h:
#define VIRTGPU_CAPSET_VIRGL 1 #define VIRTGPU_CAPSET_VIRGL2 2 #define VIRTGPU_CAPSET_GFXSTREAM_VULKAN 3 #define VIRTGPU_CAPSET_VENUS 4 #define VIRTGPU_CAPSET_CROSS_DOMAIN 5 #define VIRTGPU_CAPSET_DRM 6 #define VIRTGPU_CAPSET_GFXSTREAM_MAGMA 7 #define VIRTGPU_CAPSET_GFXSTREAM_GLES 8 #define VIRTGPU_CAPSET_GFXSTREAM_COMPOSER 9
virtio-gpu can be used to forward various things to Linux hosts, such as native Wayland surfaces, or Linux drm ioctls. Commonly this is used with Linux guests, but there's nothing preventing usage in any other OS running as guest.
Device Types
| Device ID | Device Name | Description |
|---|---|---|
| 0 | reserved (invalid) | TODO |
| 1 | network card | TODO |
| 2 | block device | TODO |
| 3 | console | TODO |
| 4 | entropy source | TODO |
| 5 | memory ballooning (traditional) | TODO |
| 6 | ioMemory | TODO |
| 7 | rpmsg | TODO |
| 8 | SCSI host | TODO |
| 9 | 9P transport | TODO |
| 10 | mac80211 wlan | TODO |
| 11 | rproc serial | TODO |
| 12 | virtio CAIF | TODO |
| 13 | memory balloon | TODO |
| 16 | GPU device | TODO |
| 17 | Timer/Clock device | TODO |
| 18 | Input device | TODO |
| 19 | Socket device | TODO |
| 20 | Crypto device | TODO |
| 21 | Signal Distribution Module | TODO |
| 22 | pstore device | TODO |
| 23 | IOMMU device | TODO |
| 24 | Memory device | TODO |
| 25 | Audio device | TODO |
| 26 | file system device | TODO |
| 27 | PMEM device | TODO |
| 28 | RPMB device | TODO |
| 29 | mac80211 hwsim wireless simulation device | TODO |
| 30 | Video encoder device | TODO |
| 31 | Video decoder device | TODO |
| 32 | SCMI device | TODO |
| 33 | NitroSecureModule | TODO |
| 34 | I2C adapter | TODO |
| 35 | Watchdog | TODO |
| 36 | CAN device | TODO |
| 38 | Parameter Server | TODO |
| 39 | Audio policy device | TODO |
| 40 | Bluetooth device | TODO |
| 41 | GPIO device | TODO |
| 42 | RDMA device | TODO |
Device Transports
VirtIO supports three different types of device transports: PCI Bus, MMIO, and Channel I/O. The PCIe transport is configured via the normal PCIe configuration mechanisms, while the MMIO transport is configured strictly via MMIO reads and writes.