mounting /mnt/vmtest with 9pfs cache=mmap option

[chantra]

It seems /mnt/vmtest mount is mounted with "cache=loose" option since ea62161

One issue with cache=loose, is that the guest does not revalidate from the host, and based on the 9p documentation it seems it should be use for RO filesystem?

https://wiki.qemu.org/Documentation/9p_root_fs#Boot_the_9p_Root_FS_System goes over this. It seems cache=mmap is the least required to have a functional system.

It becomes useful when paired with the interactive mode as this would allow, for instance, to iterate on bpf selftests (from the host) and quickly re-run them in the guest without the need to reboot. See https://chantra.github.io/bpfcitools/bpf-local-development.html

WDYT?

[danobi]

Agreed, looks like cache=loose is a bug now that it's not RO.

But it's not clear to me the difference between cache=none and cache=mmap. If mmap works for you then I suppose it's fine.

[chantra]

The wiki says:

Note: you need to use at least cache=mmap with the command above. That's actually not about performance, but rather allows the mmap() call to work on the guest system at all. Without this the guest system would even fail to boot, as many software components rely on the availability of the mmap() call.

so it seems it is desirable to enable it. I take this statement does not apply to only the rootfs.

[danobi]

Ah I see that in the qemu wiki now. Somewhat odd semantics... but sounds like cache=mmap is what we want