[libvirt] New QEMU daemon for persistent reservations

[Daniel P. Berrange]

On Tue, Oct 03, 2017 at 06:07:53PM +0200, Paolo Bonzini wrote:
> On 03/10/2017 17:59, Michal Privoznik wrote:
> > Ah, this breaks my design. I guess
> > 
> >   <disk>
> >     <pr>
> >       <source type='unix' path='/path/to/qemu-pr-helper' mode='server'/>
> >     </pr>
> >   </disk>
> > 
> > is pure madness, isn't it?
> 
> 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.

IOW, just be as simple as an attribute  "reservations=on|off" and we'll
decide the UNIX socket path, daemon forking, etc ourselves

Regards,
Daniel
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