Re: [PATCH 2/6] migration: introduce savevm, loadvm, delvm QMP commands

From: Eric Blake <eblake@redhat.com>
To: "Daniel P. Berrangé" <berrange@redhat.com>, qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>, Peter Krempa <pkrempa@redhat.com>,
	qemu-block@nongnu.org, Juan Quintela <quintela@redhat.com>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	Markus Armbruster <armbru@redhat.com>,
	Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Max Reitz <mreitz@redhat.com>
Subject: Re: [PATCH 2/6] migration: introduce savevm, loadvm, delvm QMP commands
Date: Thu, 2 Jul 2020 13:12:52 -0500	[thread overview]
Message-ID: <fcff0e8b-fd60-2897-0553-49ab24a9b7fa@redhat.com> (raw)
In-Reply-To: <20200702175754.2211821-3-berrange@redhat.com>

On 7/2/20 12:57 PM, Daniel P. Berrangé wrote:
> savevm, loadvm and delvm are some of the few commands that have never
> been converted to use QMP. The primary reason for this lack of
> conversion is that they block execution of the thread for as long as
> they run.
> 
> Despite this downside, however, libvirt and applications using libvirt
> has used these commands for as long as QMP has existed, via the
> "human-monitor-command" passthrough command. IOW, while it is clearly
> desirable to be able to fix the blocking problem, this is not an
> immediate obstacle to real world usage.
> 
> Meanwhile there is a need for other features which involve adding new
> parameters to the commands. This is possible with HMP passthrough, but
> it provides no reliable way for apps to introspect features, so using
> QAPI modelling is highly desirable.
> 
> This patch thus introduces trival savevm, loadvm, delvm commands

trivial

> to QMP that are functionally identical to the HMP counterpart, including
> the blocking problem.

Should we name them 'x-savevm', 'x-loadvm', 'x-delvm' to give ourselves 
room to change them when we DO solve the blocking issue?  Or will the 
solution of the blocking issue introduce new QMP commands, at which 
point we can add QMP deprecation markers on these commands to eventually 
retire them?

> 
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> ---

> +++ b/qapi/migration.json
> @@ -1621,3 +1621,79 @@
>   ##
>   { 'event': 'UNPLUG_PRIMARY',
>     'data': { 'device-id': 'str' } }
> +
> +##
> +# @savevm:
> +#
> +# Save a VM snapshot
> +#
> +# @tag: name of the snapshot to create. If it already
> +# exists it will be replaced.
> +#
> +# Note that execution of the VM will be paused during the time
> +# it takes to save the snapshot
> +#
> +# Returns: nothing
> +#
> +# Example:
> +#
> +# -> { "execute": "savevm",
> +#      "data": {
> +#         "tag": "my-snap"
> +#      }
> +#    }
> +# <- { "return": { } }
> +#
> +# Since: 5.2

I guess you are NOT trying to make 5.1 soft freeze next week?

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org