An io_uring-based user-space block driver (opens in new tab)

Yep! Character devices are much closer to "stream of bytes", and from the FUSE perspective they look like a single file with limited operations (open, close, read, write). Think of something like a mouse (sending a stream of motion/click events) or a webcam (send stream of frames, receive basic control commands). If you've written even the most basic FUSE layer, you've got all the necessary handlers to implement CUSE too.

Block devices operate on blocks of data identified by offset. Hard disks, CD-ROM drives, USB sticks, basically anything where it'd make sense to say "read (or write) these 1024 bytes at offset 0x10000".

You can in principle implement a block device-ish API in FUSE by disabling open/close and requiring all reads/writes to be at given offsets -- IIRC this is how the "fuseblk" mode added for ntfs-3g works -- but the protocol is too chatty to be fast enough for things people want block devices for.

I've also heard the kernel's block layer error handling doesn't interact well with the FUSE protocol, but I don't know the details too well on that.

The basis of virtio is literally just a ring queue, with its request descriptors looking almost exactly like structs suitable for passing to readv(2) or writev(2); the PCI shim is built on top of that and is completely optional (you can have a purely MMIO virtio device, after all). It was built this way so that KVM would not have to mimic the idiosyncrasies of real hardware: passing data as-is to the physical devices can't work for obvious reasons (even if you disregard security completely); instead in can, after minimal processing, shove it into the Linux kernel and let it take care of the rest.

Is there no way to avoid kernel copying all network data?

I understand the frustration of having the network driver crash but could it not be run in a way that it doesn't bring down the OS?

It seems to me Java would have a no-brainer advantage of a user-space networking option since you're already in a VM!?

When I saturate my HTTP server the kernel takes 30% of the CPU just copying data for no good reason?!

XDP is kind of the opposite of this, right? It's moving userland code into the kernel.

yes, i think there is example code where io_ring is used to get blocks into and out of XDP/kernel.

It’s obviously proprietary: it’s non-standard and specific to a single vendor.

What is the whole idea, though? Serving things to kernel from userland is decades old and commonly used with both NFS and iSCSI. The fact that this particular implementation uses io_uring instead of something non-proprietary like RDMA, is just an implementation detail.