Re: [Qemu-devel] [RFC 1/1] Execute arbitrary QMP commands from command line

[Daniel P. Berrange]

On Thu, Jan 29, 2015 at 03:22:55PM +0000, Dr. David Alan Gilbert wrote:
> * Daniel P. Berrange (address@hidden) wrote:
> > On Thu, Jan 29, 2015 at 03:06:37PM +0000, Dr. David Alan Gilbert (git) 
> > wrote:
> > > From: "Dr. David Alan Gilbert" <address@hidden>
> > > 
> > > For an incoming migration it's potentially useful to be able to set
> > > capabilities and parameters prior to opening the connection, while
> > > a separate option for that would have been possible it seems better
> > > to give access to all the existing migration capabilities, parameters
> > > etc.  The least restrictive way of doing this is to allow arbitrary
> > > QMP commands to be executed prior to the -incoming being processed.
> > > 
> > > As an example:
> > > 
> > > ./bin/qemu-system-x86_64 -nographic -nodefaults -qmp-command '{"execute": 
> > > "migrate-set-capabilities", 
> > > "arguments":{"capabilities":[{"capability":"xbzrle","state":true}]}}' 
> > > -qmp-command '{"execute": "query-migrate-capabilities"}' -incoming 
> > > tcp::444
> > 
> > I'm unclear how we'd easily deal with the response from commands
> > invoked this way, to get replies and/or errors. Also, it might
> > be the case that we need to conditionally run certain commands
> > depending on the result of earlier commands.
> > 
> > Wouldn't it make more sense to simply add a 'migrate_incoming' QMP
> > command, and stop using -incoming altogether, so we just have normal
> > QMP access ?
> > 
> > eg,
> > 
> >     #  qemu-system-x86_64 ....device args...  -S
> >     (qmp) ....arbitrary QMP commands ..
> >     (qmp) {"execute":"migrate-incoming", "arguments": { "uri": "tcp::44" }}
> 
> I'm a bit worried about whether starting an incoming migrate afterwards is
> different in any subtle way.  I can see there are a handful of devices that
> have 'runstate_check(RUN_STATE_INMIGRATE)' calls in, and thus I'm not sure
> that starting a paused VM and then flipping to RUN_STATE_INMIGRATE would
> be quite the same.
> 
> Having said that, we do have 'loadvm' that's similar to an incoming migration.

Yep, existance of 'loadvm' does give confidence that this should work with
migration, hopefully only needing a few cleanups for the state check you
mention above.


Regards,
Daniel
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