on bhyve statistics

[Fabian Freyer]

Hi list,

I'm currently looking at getting the libvirt prometheus exporter[1] to
work with libvirt+bhyve. In its current state this doesn't work because
at least one of the API calls exposed by libvirt isn't implemented by
the libvirt bhyve driver - so I started looking at implementing it.

The first API call in question is virDomainBlockStats[2], which returns
statistics (number of read and written bytes and ops, respectively).

Currently, bhyve does not expose any of these statistics. All the stats
available through bhyvectl --get-stats seem to be coming from the VMM,
not from the userspace emulation. OTOH, I did see that there are *some*
stats being collected in bhyverun.c (see struct bhyvestats {...}
stats;). I can't see how these are exposed though -  a grep of /usr/src
turned up no other uses. Which brings me to the following questions:

- are the stats in struct bhyvestats {...} stats exposed or used in any
  non-obvious way?

- architecturally, what would be the best ways to get stats out of the
  user-space emulations? Off of the top of my head, I could think of the
  following possibilities:
  - prometheus exporter
  - having some socket or pipe to request them
  - DTrace probes

I wouldn't mind implementing any of the above, and so would like to know
which of these (or other options) would be the most acceptable, and
would appreciate some guidance.

CC'ing novel@ for the libvirt side, and grehan@ for the architectural
bhyve questions.

Fabian

[1] https://github.com/kumina/libvirt_exporter
[2] https://libvirt.org/html/libvirt-libvirt-domain.html#virDomainBlockStats