Re: [PATCH v2 9/9] docs/migration: reflect the changes about needed subsections

[Marc-André Lureau]

Hi

On Tue, Oct 24, 2023 at 2:47 PM Juan Quintela <quintela@redhat.com> wrote:
>
> marcandre.lureau@redhat.com wrote:
> > From: Marc-André Lureau <marcandre.lureau@redhat.com>
> >
> > Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> > ---
> >  docs/devel/migration.rst | 17 ++++++++---------
> >  1 file changed, 8 insertions(+), 9 deletions(-)
> >
> > diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst
> > index c3e1400c0c..50f313f178 100644
> > --- a/docs/devel/migration.rst
> > +++ b/docs/devel/migration.rst
> > @@ -240,17 +240,16 @@ a newer form of device, or adding that state that you 
> > previously
> >  forgot to migrate.  This is best solved using a subsection.
> >
> >  A subsection is "like" a device vmstate, but with a particularity, it
> > -has a Boolean function that tells if that values are needed to be sent
> > -or not.  If this functions returns false, the subsection is not sent.
> > -Subsections have a unique name, that is looked for on the receiving
> > -side.
> > +has a Boolean function that tells if that values are needed or not. If
> > +this functions returns false, the subsection is not sent. Subsections
> > +have a unique name, that is looked for on the receiving side.
> >
> >  On the receiving side, if we found a subsection for a device that we
> > -don't understand, we just fail the migration.  If we understand all
> > -the subsections, then we load the state with success.  There's no check
> > -that a subsection is loaded, so a newer QEMU that knows about a subsection
> > -can (with care) load a stream from an older QEMU that didn't send
> > -the subsection.
> > +don't understand, we just fail the migration. If we understand all the
> > +subsections, then we load the state with success. There's no check
> > +that an optional subsection is loaded, so a newer QEMU that knows
> > +about a subsection can (with care) load a stream from an older QEMU
> > +that didn't send the subsection.
>
> Reviewed-by: Juan Quintela <quintela@redhat.com>
>
> Just wondering.  What device propmted you to write this series?

When I worked on ramfb, I did various testing with the subsection
handling and was surprised by the default lazy behaviour. I initially
thought it was a bug to ignore missing sections (both needed and
not-needed), then I realize from the doc that it was partially by
design. I thought it was clearer to make "needed' section actually
required on load as well. I wonder though of the potential of breakage
from old QEMU versions, how do we test cross-version migration? Do you
think also "needed" section are actually required? Perhaps we need
better wording and documentation instead...