libvfio-user ============ vfio-user is a framework that allows implementing PCI devices in userspace. Clients (such as [qemu](https://qemu.org)) talk the [vfio-user protocol](https://lists.gnu.org/archive/html/qemu-devel/2020-11/msg02458.html) over a UNIX socket to a server. This library, `libvfio-user`, provides an API for implementing such servers. [VFIO](https://www.kernel.org/doc/Documentation/vfio.txt) 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](https://www.qemu.org/docs/master/interop/vhost-user.html) 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. Currently there is one, single-threaded, application instance per device, however the application can employ any form of concurrency needed. In the future we plan to make libvfio-user multi-threaded. The library (and the protocol) are actively under development, and should not be considered a stable API or interface. Work is underway to integrate the protocol with `qemu` (as a client) and [SPDK](https://spdk.io) (on the server side, implementing a virtual NVMe controller). 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. Currently reading and writing of the memory mapped memory by the client goes undetected by `libvfio-user`, the business logic needs to poll. In the future we plan to implement a mechanism in order to provide notifications to `libvfio-user` whenever a page is written to. Interrupts ---------- Interrupts are implemented by passing the event file descriptor to `libvfio-user` and then notifying it about it. `libvfio-user` can then trigger interrupts simply by writing to it. This can be much more expensive compared to triggering interrupts from the kernel, however this performance penalty is perfectly acceptable when prototyping the functional aspect of a device driver. Building libvfio-user ===================== Build requirements: * `cmake` (v2 or above) * `apt install libjson-c-dev libcmocka-dev libssl-dev` *or* * `yum install json-c-devel libcmocka-devel openssl-devel` To build: make && make install # optional make test By default a debug build is created. To create a release build do: make BUILD_TYPE=rel The kernel headers are necessary because VFIO structs and defines are reused. Finally build your program and link with `libvfio-user.so`. Example ======= The [samples directory](./samples/) contains various libvfio-user samples. lspci ----- [lspci](./samples/lspci.c) implements an example of how to dump the PCI header of a libvfio-user device and examine it with lspci(8): # lspci -vv -F <(build/dbg/samples/lspci) 00:00.0 Non-VGA unclassified device: Device 0000:0000 Control: I/O- Mem- BusMaster- SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx- Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- SERR- [disabled] Region 1: I/O ports at [disabled] Region 2: I/O ports at [disabled] Region 3: I/O ports at [disabled] Region 4: I/O ports at [disabled] Region 5: I/O ports at [disabled] Capabilities: [40] Power Management version 0 Flags: PMEClk- DSI- D1- D2- AuxCurrent=0mA PME(D0-,D1-,D2-,D3hot-,D3cold-) Status: D0 NoSoftRst+ PME-Enable- DSel=0 DScale=0 PME- The above sample implements a very simple PCI device that supports the Power Management PCI capability. The sample can be trivially modified to change the PCI configuration space header and add more PCI capabilities. Client/Server Implementation ---------------------------- [Client](./samples/client.c)/[server](./samples/server.c) implements a basic client/server model where basic tasks are performed. The server implements a device that can be programmed to trigger interrupts (INTx) to the client. This is done by writing the desired time in seconds since Epoch to BAR0. The server then triggers an eventfd-based IRQ and then a message-based one (in order to demonstrate how it's done when passing of file descriptors isn't possible/desirable). The device also works as memory storage: BAR1 can be freely written to/read from by the host. Since this is a completely made up device, there's no kernel driver (yet). [Client](./samples/client.c) implements a client that knows how to drive this particular device (that would normally be QEMU + guest VM + kernel driver). The client excercises all commands in the vfio-user protocol, and then proceeds to perform live migration. The client spawns the destination server (this would be normally done by libvirt) and then migrates the device state, before switching entirely to the destination server. We re-use the source client instead of spawning a destination one as this is something libvirt/QEMU would normally do. To spice things up, the client programs the source server to trigger an interrupt and then migrates to the destination server; the programmed interrupt is delivered by the destination server. Also, while the device is being live migrated, the client spawns a thread that constantly writes to BAR1 in a tight loop. This thread emulates the guest VM accessing the device while the main thread (what would normally be QEMU) is driving the migration. Start the source server as follows (pick whatever you like for `/tmp/vfio-user.sock`): rm -f /tmp/vfio-user.sock* ; build/dbg/samples/server -v /tmp/vfio-user.sock And then the client: build/dbg/samples/client /tmp/vfio-user.sock After a couple of seconds the client will start live migration. The source server will exit and the destination server will start, watch the client terminal for destination server messages. gpio ---- A [gpio](./samples/gpio-pci-idio-16.c) server implements a very simple GPIO device that can be used with a Linux VM. First, download and build [this branch of qemu](https://github.com/oracle/qemu/pull/1). Start the `gpio` server process: rm /tmp/vfio-user.sock ./build/dbg/samples/gpio-pci-idio-16 -v /tmp/vfio-user.sock & Create a Linux install image, or use a pre-made one. You'll probably also need to build the `gpio-pci-idio-16` kernel module yourself - it's part of the standard Linux kernel, but not usually built and shipped on x86. Start your guest VM: ./x86_64-softmmu/qemu-system-x86_64 -mem-prealloc -m 256 \ -object memory-backend-file,id=ram-node0,prealloc=yes,mem-path=/dev/hugepages/gpio,share=yes,size=256M \ -numa node,memdev=ram-node0 \ -kernel ~/vmlinuz -initrd ~/initrd -nographic \ -append "console=ttyS0 root=/dev/sda1 single" \ -hda ~/bionic-server-cloudimg-amd64-0.raw \ -device vfio-user-pci,socket=/tmp/vfio-user.sock Log in to your guest VM, and you should be able to load the module and observe the emulated GPIO device's pins: insmod gpio-pci-idio-16.ko cat /sys/class/gpio/gpiochip480/base > /sys/class/gpio/export for ((i=0;i<12;i++)); do cat /sys/class/gpio/OUT0/value; done 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](https://libvfio-user.slack.com); or IRC at [#qemu on OFTC](https://oftc.net/). Contributing ============ Contributions are welcome; please file an [issue](https://github.com/nutanix/libvfio-user/issues/) or [open a PR](https://github.com/nutanix/libvfio-user/pulls). 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](https://en.wikipedia.org/wiki/Developer_Certificate_of_Origin). History ======= This project was formerly known as "muser", short for "Mediated Userspace Device". It implemented a proof-of-concept [VFIO mediated device](https://www.kernel.org/doc/Documentation/vfio-mediated-device.txt) 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](https://github.com/nutanix/muser/tree/kmod). License ======= Copyright © 2019-2020 Nutanix Inc. 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