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

From: Daniel P . Berrangé
Subject: Re: [Qemu-devel] [PATCH v6 03/11] cli: add --preconfig option
Date: Tue, 1 May 2018 16:07:23 +0100
User-agent: Mutt/1.9.2 (2017-12-15)

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.

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 ?

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