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

[Dr. David Alan Gilbert]

* Daniel P. Berrange (address@hidden) wrote:
> 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.

Hmm I'm not sure; I've had a dig and the tests for INMIGRATE do a lot of random
things; qxl, usb, and blockdev for starters, and the block stuff sets a flag
that is then passed to qcow and other backends that do stuff all over with it.

Howver, something like:

   -incoming pause

followed by the monitor command later should work - how do you feel
about that?

Dave

> 
> 
> Regards,
> Daniel
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--
Dr. David Alan Gilbert / address@hidden / Manchester, UK