[libvirt] [REPOST] regarding cgroup v2 support in libvirt

[Tejun Heo]

(reposting w/ libvir-list cc'd, sorry about the delay in reposting,
 was traveling and then on vacation)

Hello, Daniel.  How have you been?

We (facebook) are deploying cgroup v2 and internally use libvirt to
manage virtual machines, so I'm trying to add cgroup v2 support to
libvirt.

Because cgroup v2's resource configurations differ from v1 in varying
degrees depending on the specific resource type, it unfortunately
introduces new configurations (some completely new configs, others
just a different range / format).  This means that adding cgroup v2
support to libvirt requires adding new config options to it and maybe
implementing some form of translation mechanism between overlapping
configs.

The upcoming systemd release includes all that's necessary to support
v1/v2 compatibility so that users setting resource configs through
systemd don't have to worry about whether v1 or v2 is in use.  I'm
wondering whether it would make sense to make libvirt use dbus calls
to systemd to set resource configs when systemd is in use, so that it
can piggyback on systemd's v1/v2 compatibility.

It is true that, as libvirt can be used without systemd, libvirt will
probably want its own direct implementation down the line, but I think
there are benefits to going through systemd for resource settings in
general given that hierarchy setup is already done through systemd
when available.

What do you think?

Thanks!

-- 
tejun