Re: [Qemu-devel] [PATCH v6 03/11] cli: add --preconfig option

[Igor Mammedov <imammedo@redhat.com>]

On Tue, 1 May 2018 16:07:23 +0100
Daniel P. Berrangé <berrange@redhat.com> wrote:

> On Fri, Apr 27, 2018 at 05:05:15PM +0200, Igor Mammedov wrote:
> > This option allows pausing QEMU in the new RUN_STATE_PRECONFIG state,
> > allowing the configuration of QEMU from QMP before the machine jumps
> > into board initialization code of machine_run_board_init()
> > 
> > The intent is to allow management to query machine state and additionally
> > configure it using previous query results within one QEMU instance
> > (i.e. eliminate the need to start QEMU twice, 1st to query board specific
> > parameters and 2nd for actual VM start using query results for
> > additional parameters).
> > 
> > The new option complements -S option and could be used with or without
> > it. The difference is that -S pauses QEMU when the machine is completely
> > initialized with all devices wired up and ready to execute guest code
> > (QEMU needs only to unpause VCPUs to let guest execute its code),
> > while the "preconfig" option pauses QEMU early before board specific init
> > callback (machine_run_board_init) is executed and allows the configuration
> > of machine parameters which will be used by board init code.  
> 
> > When early introspection/configuration is done, command 'exit-preconfig'
> > should be used to exit RUN_STATE_PRECONFIG and transition to the next
> > requested state (i.e. if -S is used then QEMU will pause the second
> > time when board/device initialization is completed or start guest
> > execution if -S isn't provided on CLI)  
> 
> If we're going to introduce this --preconfig facility I would very much
> like us to ensure that we do *not* need to continue to also use -S. IOW
> it would be nice to essentially consider --preconfig a better impl of
> -S and stop using -S, perhaps even deprecating it for removal.
That's how I view this as well,
modulo removal of -S option as it still might be used for hotplugging and
for early VM startup dubugging and other thing that really need ready
to run machine.

> What logical things does QEMU do after exiting pre-config stage, but
> before starting CPUs that might be dependency for mgmt apps that would
> neccessitate using both --preconfig and -S ?
I've replied to your other email listing them, so lets figure
out the plan of what needs to be fixed so that libvirt could
switch from -S to --preconfig only.
PS:
(Since I've started this whole thing, I'm willing to work on
gradually improving it)

> Regards,
> Daniel