I want to continue the discussion regarding using MUSER
(https://github.com/nutanix/muser) as a device offloading mechanism. The main
drawback of MUSER is that it requires a kernel module, so I've experimented
with a proof of concept of how MUSER would look like if we somehow didn't need
a kernel module. I did this by implementing a wrapper library
(https://github.com/tmakatos/libpathtrap) that intercepts accesses to
VFIO-related paths and forwards them to the MUSER process providing device
emulation over a UNIX domain socket. This does not require any changes to QEMU
(4.1.0). Obviously this is a massive hack and is done only for the needs of
this PoC.

The result is a fully working PCI device in QEMU (the gpio sample explained in
which is as simple as possible. I've also tested with a much more complicated
device emulation, https://github.com/tmakatos/spdk, which provides NVMe device
emulation and requires accessing guest memory for DMA, allowing BAR0 to be
memory mapped into the guest, using MSI-X interrupts, etc.

The changes required in MUSER are fairly small, all that is needed is to
introduce a new concept of "transport" to receive requests from a UNIX domain
socket instead of the kernel (from a character device) and to send/receive file
descriptors for sharing memory and firing interrupts.

My experience is that VFIO is so intuitive to use for offloading device
emulation from one process to another that makes this feature quite
straightforward. There's virtually nothing specific to the kernel in the VFIO
API. Therefore I strongly agree with Stefan's suggestion to use it for device
offloading when interacting with QEMU. Using 'muser.ko' is still interesting
when QEMU is not the client, but if everyone is happy to proceed with the
vfio-over-socket alternative the kernel module can become a second-class
citizen. (QEMU is, after all, our first and most relevant client.)

Next I explain how to test the PoC.

Build MUSER with vfio-over-socket:

        git clone --single-branch --branch vfio-over-socket 
        cd muser/
        git submodule update --init

Run device emulation, e.g.

        ./build/dbg/samples/gpio-pci-idio-16 -s <N>

Where <N> is an available IOMMU group, essentially the device ID, which must not
previously exist in /dev/vfio/.

Run QEMU using the vfio wrapper library and specifying the MUSER device:

        LD_PRELOAD=muser/build/dbg/libvfio/libvfio.so qemu-system-x86_64 \
                ... \
                -device vfio-pci,sysfsdev=/dev/vfio/<N> \
                -numa node,nodeid=0,cpus=0,memdev=ram-node0

Bear in mind that since this is just a PoC lots of things can break, e.g. some
system call not intercepted etc.

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