9p filesystem has been added to FreeBSD

For a read-only filesystem, as a shared directory between guest and host OS for a specific guest (and host) users, etc. It's a significant, potentially dangerous limitation, but it's still worth having in its current form. Touching the VFS interface is easier when the code consuming it is already in the tree and not to be taken lightly.

My impression is that it really matters which 9p implementation you're using - for example, it's used in both WSL2 and ChromeOS's VM system, and it seems quite good there. Though equally I'd easily believe that there are terrible implementations running around.

It's a virtio filesystem, so it sends 9p.2000L (probably) down a virtio "pipe" so your virtual machine can access its host filesystem

9p is a simple network protocol for implementing filesystems. It originates from the plan9 operating system but its main usefulness nowadays is that it permits people from implementing filesystems in user space without having to integrate deeply into kernels specific interfaces like FUSE.

I.e. if your OS can mount 9p volumes you can implement a user filesystem easily by just running some process on localhost and mount it. And it also works remotely (although that's less useful IMO, although not entirely useless)

Bhyve (FreeBSD's native hypervisor) already contains a 9P over VirtIO "server". The protocols allows virtual machines and their host to share a directory instead of block storage without having to configure and use a network filesystem like NFS. It's even bootable as in the UEFI BootROM should be able to read the bootloader via 9P, the bootloader can use EFI boot services to read the kernel (configuration, kernel image, kernel modules), and the kernel can than mount it as root filesystem. This means that use a directory instead of an image to boot a virtual machine. I can imagine a lot of use-cases for this e.g. a CI/CD pipeline that tests an experimental kernel (feature) inside a hypervisor.