The API is currently documented via the libvfio-user header file, along with some additional documentation.
The library is actively under development, and should not yet be considered a stable API/ABI.
The protocol itself can be considered stable and will not break backwards compatibility. See the QEMU repository for the canonical protocol definition.
The API is not thread safe, but individual vfu_ctx_t handles can be used separately by each thread: that is, there is no global library state.
See Accessing memory with libvfio-user for more details on how to manage memory.
See Examples for some simple examples of using the library.
With the client support found in cloud-hypervisor or qemu, most guest VM use cases will work.
However, guests with an IOMMU (vIOMMU) will not currently work: the number of DMA regions is strictly limited, and there are also issues with some server implementations such as SPDK's virtual NVMe controller.
Currently, libvfio-user has explicit support for PCI devices only. In addition, only PCI endpoints are supported (no bridges etc.).
The master branch of libvfio-user implements live migration with a protocol based on vfio's v2 protocol. Currently, there is no support for this in any qemu client. Contributions are welcome!