Some of my colleagues have been working on the multiqueue I/O support for a few years now. It's only ~ 10 years ago that qemu handled all I/O on a single thread (using coroutines). Getting from there to now where I/O can be handled by multiple threads, hence multiple host cores, has been a huge task in a code base that's so large and over 20 years old.

Talk from Kevin Wolf about this: https://kvm-forum.qemu.org/2023/Multiqueue_in_the_block_laye...