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-devoryum 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.