Re: [PATCH v3 3/8] migration/savevm: Allow immutable device state to be migrated early (i.e., before RAM)

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* David Hildenbrand (david@redhat.com) wrote:
> For virtio-mem, we want to have the plugged/unplugged state of memory
> blocks available before migrating any actual RAM content, and perform
> sanity checks before touching anything on the destination. This
> information is immutable on the migration source while migration is active,
> 
> We want to use this information for proper preallocation support with
> migration: currently, we don't preallocate memory on the migration target,
> and especially with hugetlb, we can easily run out of hugetlb pages during
> RAM migration and will crash (SIGBUS) instead of catching this gracefully
> via preallocation.
> 
> Migrating device state via a vmsd before we start iterating is currently
> impossible: the only approach that would be possible is avoiding a vmsd
> and migrating state manually during save_setup(), to be restored during
> load_state().
> 
> Let's allow for migrating device state via a vmsd early, during the
> setup phase in qemu_savevm_state_setup(). To keep it simple, we
> indicate applicable vmds's using an "immutable" flag.
> 
> Note that only very selected devices (i.e., ones seriously messing with
> RAM setup) are supposed to make use of such early state migration.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  include/migration/vmstate.h |  5 +++++
>  migration/savevm.c          | 14 ++++++++++++++
>  2 files changed, 19 insertions(+)
> 
> diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h
> index ad24aa1934..dd06c3abad 100644
> --- a/include/migration/vmstate.h
> +++ b/include/migration/vmstate.h
> @@ -179,6 +179,11 @@ struct VMStateField {
>  struct VMStateDescription {
>      const char *name;
>      int unmigratable;
> +    /*
> +     * The state is immutable while migration is active and is saved
> +     * during the setup phase, to be restored early on the destination.
> +     */
> +    int immutable;

A bool would be nicer (as it would for unmigratable above)

>      int version_id;
>      int minimum_version_id;
>      MigrationPriority priority;
> diff --git a/migration/savevm.c b/migration/savevm.c
> index ff2b8d0064..536d6f662b 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -1200,6 +1200,15 @@ void qemu_savevm_state_setup(QEMUFile *f)
>  
>      trace_savevm_state_setup();
>      QTAILQ_FOREACH(se, &savevm_state.handlers, entry) {
> +        if (se->vmsd && se->vmsd->immutable) {
> +            ret = vmstate_save(f, se, ms->vmdesc);
> +            if (ret) {
> +                qemu_file_set_error(f, ret);
> +                break;
> +            }
> +            continue;
> +        }
> +

Does this give you the ordering you want? i.e. there's no guarantee here
that immutables come first?

Dave


>          if (!se->ops || !se->ops->save_setup) {
>              continue;
>          }
> @@ -1402,6 +1411,11 @@ int qemu_savevm_state_complete_precopy_non_iterable(QEMUFile *f,
>      int ret;
>  
>      QTAILQ_FOREACH(se, &savevm_state.handlers, entry) {
> +        if (se->vmsd && se->vmsd->immutable) {
> +            /* Already saved during qemu_savevm_state_setup(). */
> +            continue;
> +        }
> +
>          ret = vmstate_save(f, se, vmdesc);
>          if (ret) {
>              qemu_file_set_error(f, ret);
> -- 
> 2.39.0
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK