Re: [PATCH RFC 0/2] migration/vl: new -incoming config:* for early migration parameters

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Jun 03, 2026 at 11:50:53AM -0400, Peter Xu wrote:
> On Wed, Jun 03, 2026 at 10:48:54AM +0100, Daniel P. Berrangé wrote:
> > On Fri, May 29, 2026 at 10:15:35AM -0400, Peter Xu wrote:
> > > On Thu, May 28, 2026 at 07:01:50PM -0300, Fabiano Rosas wrote:
> > > > Peter Xu <peterx@redhat.com> writes:
> > > > 
> > > > > CI: https://gitlab.com/peterx/qemu/-/pipelines/2560168907
> > > > >
> > > > > This series introduces a generic way to specify migration parameters that
> > > > > can be used even during the early boot phase of QEMU.
> > > > >
> > > > > One example use case that already existed is CPR-transfer / CPR-exec.
> > > > > currently QEMU has a temporary global variable (incoming_mode) to achieve
> > > > > this, but it's hard to understand and this hack bleeded into quite a few
> > > > > places that we could have avoided.  The lines in patch 2 touched may
> > > > > provide some idea.
> > > > >
> > > > > With a generic approach of setting migration parameters with cmdlines, we
> > > > > can remove this hack meanwhile QEMU should be able to keep the CPR behavior
> > > > > as before.  To CPR maintainers and reviewers: please have a closer look,
> > > > > even better if it can be smoke tested, to see if this works for Oracle's
> > > > > environment, TIA.
> > > > >
> > > > > The 1st patch implemented that new semantics.  It is straightforward: now
> > > > > we can setup any migration parameter using an extra line of:
> > > > >
> > > > >   -incoming config:key1=value1,key2=value2,...

snip

> > > > - using "config:"
> > > > 
> > > > This makes it non-uniform with uri and channels which don't have a
> > > > keyword in front of them. I guess I could live with it, but it seems
> > > > odd. I see that it makes parsing way easier.
> > > 
> > > Yeah, having some identifier would be nice.  I wished channels also have
> > > identifiers if we don't need to keep compatibility.
> > 
> > IMHO having a magic "config:" prefix is an anti-pattern because it
> > involves custom command line parsing logic.
> 
> Yes, it's not elegant.  I wished we had something like this though when
> introducing the cpr channels; right now anything wasn't "defer" or URI
> implies it's a "channel"..
> 
> I also wanted to avoid introducing new cmdline parameters, so migration
> incoming cmdlines can stick with the same option.  If there's better
> suggestion please shoot.
> 
> > 
> > Does "-incoming config" imply '-incoming defer' semantics or is it
> > independent ?  
> 
> They're independent.  Examples:
> 
>  a) "-incoming config:* -incoming tcp:*", setup parameters for TCP incoming
>     migration without further deferral
> 
>  b) "-incoming config:*" only, setup global parameters for a possible
>     upcoming outgoing migration

In that case they should definitely be independent command line
options. The old "-incoming" design is already broken / limited
in the non-'defer' case because it is hardcoded to use URI syntax
which can't express all the address formats we accept in QAPI
syntax. Adding more special cases onto -incoming just makes the
bad situation even worse and is not a forward looking design.

If we need the ability to specify migration parameters on the
CLI, then as a starting point for the design, we should assume that
-incoming does not exist, and design a complete solution from scratch
that uses QAPI exclusively, both for addresses and configuration.

As discussed before, IMHO the "migrate" and "migrate-incoming"
commands need to accept both the address(s) and parameters/capabilities
as inline data items rather than relying on pre-configured global state
from the 'migrate-parameter' / 'migrate-capability' commands.

If we did that modelling for 'migrate-incoming' then that modelling of
command parmaeters could map directly to a new '-migrate-incoming'
command line argument that accepted exactly the same data model.

With regards,
Daniel
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