Re: [Xen-devel] [RFC 4/4] HACK libxl_exec: Check QEMU status via QMP instead of xenstore

[Ian Jackson <ian.jackson@xxxxxxxxxx>]

Anthony PERARD writes ("[RFC 4/4] HACK libxl_exec: Check QEMU status via QMP 
instead of xenstore"):
> This path is more of a prof of concept reather than a patch as this
> would break qemu-trad.
...
> For libxl, the only way to find out if qemu is ready on migrate/restore,
> it is to connect to the QMP socket and run "query-status".
> 
> This patch succeed in implementing that, but QMP doesn't fit well with
> the libxl__ev_* infrastructure. One main issue would be qmp_open(), it
> tries to connect to the QMP socket during 5 seconds without ever giving
> back the hand to libxl.

There are two problems here, I think.  The first one is an internal
libxl api issue: ie, that the libxl qmp code does not have the proper
callback-style API.  That can be fixed inside libxl, although it's
probably annoying.

The second is that AFAICT there is no way other than xenstore to get a
notification of any kind when qemu is ready.  So the only possible
approach is polling.  That's pretty nasty.  I haven't looked at the
qemu code in detail to check if this is really true.  Perhaps looking
at libvirt would give us a clue...

Ian.

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