Hi On Tue, Oct 24, 2023 at 2:47 PM Juan Quintela <quint...@redhat.com> wrote: > > marcandre.lur...@redhat.com wrote: > > From: Marc-André Lureau <marcandre.lur...@redhat.com> > > > > Signed-off-by: Marc-André Lureau <marcandre.lur...@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 <quint...@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...
