Re: [libvirt] New QEMU daemon for persistent reservations
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'?...
Paolo
(*) I might say, I have some reservations about it
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