Re: [PATCH v4 1/8] migration/vmstate: introduce vmstate_{load, save}_field helpers

[Fabiano Rosas <farosas@suse.de>]

Alexander Mikhalitsyn via qemu development <qemu-devel@nongnu.org>
writes:

> Am Fr., 13. März 2026 um 17:27 Uhr schrieb Peter Xu <peterx@redhat.com>:
>>
>> On Fri, Mar 13, 2026 at 03:12:14PM +0100, Alexander Mikhalitsyn wrote:
>> > From: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@futurfusion.io>
>> >
>> > Let's introduce vmstate_{load,save}_field() helpers, they will
>> > be used in next patches to support fully-dynamic arrays with NULLs.
>> >
>> > Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@futurfusion.io>
>>
>
> Hi Peter,
>
>> Feel free to rebase to Vladimir's series here, Fabiano already queued it:
>>
>> https://lore.kernel.org/all/20260304212303.667141-11-vsementsov@yandex-team.ru/#t
>
> I've tried, but I can't see it anywhere
> https://gitlab.com/farosas/qemu/-/commits/migration-next
> doesn't contain it. As of now, last commit is
> 472f455cfbb0c5f84a7d48228d5226eaf6c7a61b.
>

I think we usually just take the patches from the list and apply
directly instead of taking any branch*. I don't keep migration-next
stable.

I pushed migration-staging which you can use. It'll collect the patches
that will be merged at the start of the next development cycle, after
the release.

* I use:
b4 am -3 -c -o <patches/dir> <lore.kernel.org link>
(you'd need to solve any conflicts with the base branch yourself)

> Also, patchew tells that that series has conflicts with master:
> https://patchew.org/QEMU/20260304212303.667141-1-vsementsov@yandex-team.ru/
>
> Should I manually download mbox, apply/resolve conflicts with master,
> then rebase my stuff and resend?
>
> Kind regards,
> Alex
>
>>
>> These codes weren't touched that frequent, it's interesting to see such
>> coinsidence happens. :)
>>
>> > ---
>> >  include/migration/vmstate.h |  4 ++
>> >  migration/vmstate.c         | 84 +++++++++++++++++++++++--------------
>> >  2 files changed, 56 insertions(+), 32 deletions(-)
>> >
>> > diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h
>> > index 62c2abd0c49..5d7dfe70643 100644
>> > --- a/include/migration/vmstate.h
>> > +++ b/include/migration/vmstate.h
>> > @@ -1240,8 +1240,12 @@ extern const VMStateInfo vmstate_info_qlist;
>> >          .flags = VMS_END, \
>> >      }
>> >
>> > +int vmstate_load_field(QEMUFile *f, void *pv, size_t size,
>> > +                       const VMStateField *field, Error **errp);
>> >  int vmstate_load_state(QEMUFile *f, const VMStateDescription *vmsd,
>> >                         void *opaque, int version_id, Error **errp);
>> > +int vmstate_save_field(QEMUFile *f, void *pv, size_t size,
>> > +                       const VMStateField *field, JSONWriter *vmdesc, Error **errp);
>> >  int vmstate_save_state(QEMUFile *f, const VMStateDescription *vmsd,
>> >                         void *opaque, JSONWriter *vmdesc, Error **errp);
>> >  int vmstate_save_state_v(QEMUFile *f, const VMStateDescription *vmsd,
>> > diff --git a/migration/vmstate.c b/migration/vmstate.c
>> > index 4d28364f7ba..8d192bcaa27 100644
>> > --- a/migration/vmstate.c
>> > +++ b/migration/vmstate.c
>> > @@ -131,6 +131,32 @@ static void vmstate_handle_alloc(void *ptr, const VMStateField *field,
>> >      }
>> >  }
>> >
>> > +int vmstate_load_field(QEMUFile *f, void *pv, size_t size,
>> > +                       const VMStateField *field, Error **errp)
>> > +{
>> > +    int ret = 0;
>> > +
>> > +    if (field->flags & VMS_STRUCT) {
>> > +        ret = vmstate_load_state(f, field->vmsd, pv,
>> > +                                 field->vmsd->version_id,
>> > +                                 errp);
>> > +    } else if (field->flags & VMS_VSTRUCT) {
>> > +        ret = vmstate_load_state(f, field->vmsd, pv,
>> > +                                 field->struct_version_id,
>> > +                                 errp);
>> > +    } else {
>> > +        ret = field->info->get(f, pv, size, field);
>> > +        if (ret < 0) {
>> > +            error_setg(errp,
>> > +                       "Failed to load element of type %s for %s: "
>> > +                       "%d", field->info->name,
>> > +                       field->name, ret);
>> > +        }
>> > +    }
>> > +
>> > +    return ret;
>> > +}
>> > +
>> >  int vmstate_load_state(QEMUFile *f, const VMStateDescription *vmsd,
>> >                         void *opaque, int version_id, Error **errp)
>> >  {
>> > @@ -203,24 +229,7 @@ int vmstate_load_state(QEMUFile *f, const VMStateDescription *vmsd,
>> >                      inner_field = field;
>> >                  }
>> >
>> > -                if (inner_field->flags & VMS_STRUCT) {
>> > -                    ret = vmstate_load_state(f, inner_field->vmsd, curr_elem,
>> > -                                             inner_field->vmsd->version_id,
>> > -                                             errp);
>> > -                } else if (inner_field->flags & VMS_VSTRUCT) {
>> > -                    ret = vmstate_load_state(f, inner_field->vmsd, curr_elem,
>> > -                                             inner_field->struct_version_id,
>> > -                                             errp);
>> > -                } else {
>> > -                    ret = inner_field->info->get(f, curr_elem, size,
>> > -                                                 inner_field);
>> > -                    if (ret < 0) {
>> > -                        error_setg(errp,
>> > -                                   "Failed to load element of type %s for %s: "
>> > -                                   "%d", inner_field->info->name,
>> > -                                   inner_field->name, ret);
>> > -                    }
>> > -                }
>> > +                ret = vmstate_load_field(f, curr_elem, size, inner_field, errp);
>> >
>> >                  /* If we used a fake temp field.. free it now */
>> >                  if (inner_field != field) {
>> > @@ -427,6 +436,29 @@ int vmstate_save_state(QEMUFile *f, const VMStateDescription *vmsd,
>> >      return vmstate_save_state_v(f, vmsd, opaque, vmdesc_id, vmsd->version_id, errp);
>> >  }
>> >
>> > +int vmstate_save_field(QEMUFile *f, void *pv, size_t size,
>> > +                       const VMStateField *field, JSONWriter *vmdesc, Error **errp)
>> > +{
>> > +    int ret = 0;
>> > +
>> > +    if (field->flags & VMS_STRUCT) {
>> > +        ret = vmstate_save_state(f, field->vmsd, pv, vmdesc, errp);
>> > +    } else if (field->flags & VMS_VSTRUCT) {
>> > +        ret = vmstate_save_state_v(f, field->vmsd, pv, vmdesc,
>> > +                                   field->struct_version_id, errp);
>> > +    } else {
>> > +        ret = field->info->put(f, pv, size, field, vmdesc);
>> > +        if (ret < 0) {
>> > +            error_setg(errp,
>> > +                       "Failed to save element of type %s for %s: "
>> > +                       "%d", field->info->name,
>> > +                       field->name, ret);
>> > +        }
>> > +    }
>> > +
>> > +    return ret;
>> > +}
>> > +
>> >  int vmstate_save_state_v(QEMUFile *f, const VMStateDescription *vmsd,
>> >                           void *opaque, JSONWriter *vmdesc, int version_id, Error **errp)
>> >  {
>> > @@ -528,19 +560,7 @@ int vmstate_save_state_v(QEMUFile *f, const VMStateDescription *vmsd,
>> >                  vmsd_desc_field_start(vmsd, vmdesc_loop, inner_field,
>> >                                        i, max_elems);
>> >
>> > -                if (inner_field->flags & VMS_STRUCT) {
>> > -                    ret = vmstate_save_state(f, inner_field->vmsd,
>> > -                                             curr_elem, vmdesc_loop, errp);
>> > -                } else if (inner_field->flags & VMS_VSTRUCT) {
>> > -                    ret = vmstate_save_state_v(f, inner_field->vmsd,
>> > -                                               curr_elem, vmdesc_loop,
>> > -                                               inner_field->struct_version_id,
>> > -                                               errp);
>> > -                } else {
>> > -                    ret = inner_field->info->put(f, curr_elem, size,
>> > -                                                 inner_field, vmdesc_loop);
>> > -                }
>> > -
>> > +                ret = vmstate_save_field(f, curr_elem, size, inner_field, vmdesc_loop, errp);
>> >                  written_bytes = qemu_file_transferred(f) - old_offset;
>> >                  vmsd_desc_field_end(vmsd, vmdesc_loop, inner_field,
>> >                                      written_bytes);
>> > @@ -551,7 +571,7 @@ int vmstate_save_state_v(QEMUFile *f, const VMStateDescription *vmsd,
>> >                  }
>> >
>> >                  if (ret) {
>> > -                    error_setg(errp, "Save of field %s/%s failed",
>> > +                    error_prepend(errp, "Save of field %s/%s failed: ",
>> >                                  vmsd->name, field->name);
>> >                      if (vmsd->post_save) {
>> >                          vmsd->post_save(opaque);
>> > --
>> > 2.47.3
>> >
>>
>> --
>> Peter Xu
>>