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Re: [PATCH] migration: Unify and trace vmstate field_exists() checks
From: |
Peter Xu |
Subject: |
Re: [PATCH] migration: Unify and trace vmstate field_exists() checks |
Date: |
Tue, 3 Oct 2023 11:04:51 -0400 |
On Thu, Sep 07, 2023 at 08:55:52AM -0300, Fabiano Rosas wrote:
> Peter Xu <peterx@redhat.com> writes:
>
> > For both save/load we actually share the logic on deciding whether a field
> > should exist. Merge the checks into a helper and use it for both save and
> > load. When doing so, add documentations and reformat the code to make it
> > much easier to read.
> >
> > The real benefit here (besides code cleanups) is we add a trace-point for
> > this; this is a known spot where we can easily break migration
> > compatibilities between binaries, and this trace point will be critical for
> > us to identify such issues.
> >
> > For example, this will be handy when debugging things like:
> >
> > https://gitlab.com/qemu-project/qemu/-/issues/932
> >
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> > migration/vmstate.c | 34 ++++++++++++++++++++++++++--------
> > migration/trace-events | 1 +
> > 2 files changed, 27 insertions(+), 8 deletions(-)
> >
> > diff --git a/migration/vmstate.c b/migration/vmstate.c
> > index 31842c3afb..73e74ddea0 100644
> > --- a/migration/vmstate.c
> > +++ b/migration/vmstate.c
> > @@ -25,6 +25,30 @@ static int vmstate_subsection_save(QEMUFile *f, const
> > VMStateDescription *vmsd,
> > static int vmstate_subsection_load(QEMUFile *f, const VMStateDescription
> > *vmsd,
> > void *opaque);
> >
> > +/* Whether this field should exist for either save or load the VM? */
> > +static bool
> > +vmstate_field_exists(const VMStateDescription *vmsd, const VMStateField
> > *field,
> > + void *opaque, int version_id)
> > +{
> > + bool result;
> > +
> > + if (field->field_exists) {
> > + /* If there's the function checker, that's the solo truth */
> > + result = field->field_exists(opaque, version_id);
> > + trace_vmstate_field_exists(vmsd->name, field->name,
> > field->version_id,
> > + version_id, result);
> > + } else {
> > + /*
> > + * Otherwise, we only save/load if field version is same or older.
> > + * For example, when loading from an old binary with old version,
> > + * we ignore new fields with newer version_ids.
> > + */
> > + result = field->version_id <= version_id;
>
> This one doesn't get a trace?
Right, I didn't add that since I found mostly the bug comes from
field_exists() returning different values for different qemu binaries.
So I explicitly didn't include that otherwise we'll see tons of entries
under that regard but is probably safe.
>
> Aside from that:
>
> Reviewed-by: Fabiano Rosas <farosas@suse.de>
Thanks!
--
Peter Xu
- Re: [PATCH] migration: Unify and trace vmstate field_exists() checks,
Peter Xu <=