On Fri, May 29, 2026 at 10:19:22AM +0300, Vladimir Sementsov-Ogievskiy wrote:
> On 29.05.26 00:29, Peter Xu wrote:
> > QEMU doesn't yet have a good way to specify migration parameters so that
> > they can be available even during early stage of QEMU boots. It is because
> > the migration object (who owns the migration parameters) will only be
> > created after PHASE_LATE_BACKENDS_CREATED. It means anything before it
> > reading migration parameters is illegal.
> >
> > However, QEMU does have special use cases for such, namely only-migratable
> > flag, and cpr-transfer. Recently, we have one more possible user to read a
> > to-be-introduced new migration parameters during backend initialization
> > phase. We can introduce yet another global variable (or per-device
> > parameter) to bypass this limitation, but we can also seek for a generic
> > solution that we can setup migration parameters very early, even during
> > backend initializations.
> >
> > See this discussion for more details on the context of the problem:
> >
> > https://lore.kernel.org/r/[email protected]
> >
> > This patch wants to take the latter approach.
> >
> > As a start, introduce a new way to specify migration parameters in QEMU
> > boot commandline, as proposed in the above discussion:
> >
> > -incoming config:key1=value1,key2=value2,...
>
> Do I understand correctly, that with qobject_input_visitor_new_str(),
> it automatically suuport also json-like syntax
>
> -incoming config:'{"key1": "value1", "key2": {some more complex
> structure}}'
>
> ?
Yes.
>
> >
> > When specified, QEMU will parse a string formatted MigrationParameters and
> > keep it. When migration object is created, it will apply all the settings
> > as initial value.
> >
> > Since the application of boot parameters will be after object_new()
> > completes, it means it happens after all machine compat properties or
> > -global settings (which should be done during instance_post_init()).
>
> [..]
>
> > to issuing the migrate\_incoming to allow the migration to begin.
> > +
> > +``-incoming config:key1=value1[,key2=value2,...]``
> > +
> > + Specify migration parameters in QEMU commandlines, so that these
> > + parameters will be available even during very early boot of QEMU.
> > + They will be applied properly after QEMU boots and when the migration
> > + core is initialized. From that POV, it has similar effect as setting
> > + these parameters using QMP command ``migrate-set-parameters`` or HMP
> > + command ``migrate_set_parameter``.
> > +
> > ERST
> > DEF("only-migratable", 0, QEMU_OPTION_only_migratable, \
>
> So, to have a "defer" migration with preset parameter, I need two options
>
> -incoming defer -incoming config:key=val
>
> right?
Yes.
--
Peter Xu
