Re: Is anyone working on VirtFS (FUSE over VirtIO)

On 31 Dec 2023, at 16:19, Warner Losh <imp@bsdimp.com> wrote:
> 
> Yea. The FUSE protocol is going to be the challenge here. For this to be useful, the VirtioFS support on the FreeBSD  needs to be 100% in the kernel, since you can't have userland in the loop. This isn't so terrible, though, since our VFS interface provides a natural breaking point for converting the requests into FUSE requests. The trouble, I fear, is a mismatch between FreeBSD's VFS abstraction layer and Linux's will cause issues (many years ago, the weakness of FreeBSD VFS caused problems for a company doing caching, though things have no doubt improved from those days). Second, there's a KVM tie-in for the direct mapped pages between the VM and the hypervisor. I'm not sure how that works on the client (FreeBSD) side (though the description also says it's mapped via a PCI bar, so maybe the VM OS doesn't care).

From what I can tell from a little bit of looking at the code, our FUSE implementation has a fairly cleanly abstracted layer (in fuse_ipc.c) for handling the message queue.  For VirtioFS, it would 'just' be necessary to factor out the bits here that do uio into something that talked to a VirtIO ring.  I don’t know what the VFS limitations are, but since the protocol for VirtioFS is the kernel <-> userspace protocol for FUSE, it seems that any functionality that works with FUSE filesystems in userspace would work with VirtioFS filesystems.

The shared buffer cache bits are nice, but are optional, so could be done in a later version once the basic functionality worked.  

David