In case anyone is wondering?
9p is from Plan9
Quote:
* Windows Subsystem for Linux (WSL): since Windows 10 version 1903, the subsystem implements 9P as a server and the host Windows operating system acts as a client.
* Crostini: a custom 9P server is used to provide access to files outside of a Linux VM.
* QEMU: the VirtFS device allows for filesystem sharing over 9P, which is accelerated with kernel drivers and shared memory
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https://en.wikipedia.org/wiki/9P_(protocol)
So perhaps testing is also possible with WSL and Linux Calibre on WSL? Or maybe I'm writing rubbish.
Crostini is the Linux based on Debiam 12 in the "penguin" container on a ChromeBook, which works better than I thought it would.