vfio-user is a framework that allows implementing PCI devices in userspace. Clients (such as qemu) talk the vfio-user protocol over a UNIX socket to a server. This library, libvfio-user, provides an API for implementing such servers.

VFIO is a kernel facility for providing secure access to PCI devices in userspace (including pass-through to a VM). With vfio-user, instead of talking to the kernel, all interactions are done in userspace, without requiring any kernel component; the kernel VFIO implementation is not used at all for a vfio-user device.

Put another way, vfio-user is to VFIO as vhost-user is to vhost.

The vfio-user protocol is intentionally modelled after the VFIO ioctl() interface, and shares many of its definitions. However, there is not an exact equivalence: for example, IOMMU groups are not represented in vfio-user.

There many different purposes you might put this library to, such as prototyping novel devices, testing frameworks, implementing alternatives to qemu's device emulation, adapting a device class to work over a network, etc.

The library abstracts most of the complexity around representing the device. Applications using libvfio-user provide a description of the device (eg. region and IRQ information) and as set of callbacks which are invoked by libvfio-user when those regions are accessed.

Memory Mapping the Device

The device driver can allow parts of the virtual device to be memory mapped by the virtual machine (e.g. the PCI BARs). The business logic needs to implement the mmap callback and reply to the request passing the memory address whose backing pages are then used to satisfy the original mmap call; more details here.

Interrupts

Interrupts are implemented via eventfd's passed from the client and registered with the library. libvfio-user consumers can then trigger interrupts by writing to the eventfd.

Building libvfio-user

Build requirements:

• meson (v0.53.0 or above)
• apt install libjson-c-dev libcmocka-dev or
• yum install json-c-devel libcmocka-devel

The kernel headers are necessary because VFIO structs and defines are reused.

To build:

meson build
ninja -C build

Finally build your program and link with libvfio-user.so.

Using the library

qemu

Step-by-step instructions for using libvfio-user with qemu can be found here.

See also libvirt.

SPDK

SPDK uses libvfio-user to implement a virtual NVMe controller: see SPDK and libvfio-user for more details.

Developing with the library

See Developing with libvfio-user.

Mailing List & Chat

libvfio-user development is discussed in libvfio-user-devel@nongnu.org. Subscribe here: https://lists.gnu.org/mailman/listinfo/libvfio-user-devel.

We are on Slack at libvfio-user.slack.com (invite link); or IRC at #qemu on OFTC.

Contributing

Contributions are welcome; please file an issue or open a PR. Anything substantial is worth discussing with us first.

Please make sure to mark any commits with Signed-off-by (git commit -s), which signals agreement with the Developer Certificate of Origin v1.1.

Running make pre-push will do the same checks as done in github CI. After merging, a Coverity scan is also done.

See Testing for details on how the library is tested.

History

This project was formerly known as "muser", short for "Mediated Userspace Device". It implemented a proof-of-concept VFIO mediated device in userspace. Normally, VFIO mdev devices require a kernel module; muser implemented a small kernel module that forwarded onto userspace. The old kernel-module-based implementation can be found in the kmod branch.