[libvirt] New QEMU daemon for persistent reservations

[Daniel P. Berrange]

On Tue, Oct 03, 2017 at 06:35:03PM +0200, Paolo Bonzini wrote:
> On 03/10/2017 18:17, Daniel P. Berrange wrote:
> > On Tue, Oct 03, 2017 at 06:07:53PM +0200, Paolo Bonzini wrote:
> >> Yes, but OTOH if libvirtd starts the daemon, nobody cares about the
> >> source type, so perhaps
> >>
> >>    <pr driver='helper' mode='shared'>
> >>      <source ... />
> >>    </pr>
> >>
> >> (mandatory source) vs.
> >>
> >>    <pr driver='helper' mode='private'>
> >>      <path>/path/to/qemu-pr-helper</path>
> >>    </pr>
> >>
> >> (optional path, default from global configuration) vs.
> >>
> >>    <pr driver='passthrough'/>
> >>
> >> (no helper)?
> > 
> > I tend to think we should not expose the concept of a 'helper' at all in
> > the XML. The fact that QEMU has a separate binary to do this is really an
> > internal implementation detail due to the need for privilege separation/
> > elevation.
> > 
> > Libvirt should just do the right thing running the helper with a suitable
> > configuration when needed, just like we run the TAP device helper when
> > needed.
> 
> I agree we don't need the helper path.  I am not sure about the socket
> path, but maybe that could also be in qemu.conf.  However,
> reservations='off' doesn't always make sense:
> 
> 1) if you have appropriate privileges (via unpriv_sgio on distros that
> have it, or capabilities, or just because you're using the libiscsi
> driver) you will be able to access them anyway;
> 
> 2) if you're on a multipath device, you need to use the helper anyway to
> get the right semantics;
> 
> And later on we might have other ways to implement persistent
> reservations in QEMU.  So while I'm not a big fan(*) of the
> driver='helper' moniker, I don't think an attribute is enough.  Maybe
> driver='external'?...

Yes, if there's a choice of ways to manage reservations, we could
reflect that as  'reservations=passthrough|emulated' or something
like that. 

I just don't think the concept of a helper program should be visible
in the XML, as it is an impl detail that is totally QEMU specific and
could conceivably change eg not even needed with unpriv_sgio, and if
kernel were enhanced could be usable without needing a helper elsewhere
too.

Regards,
Daniel
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