Re: [PATCH v5 3/8] migration/savevm: Allow immutable device state to be migrated early (i.e., before RAM)

[Juan Quintela <quintela@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 VMSD's using an "early_setup" 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.
>
> While at it, also use a bool for the "unmigratable" member.
>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Reviewed-by: Juan Quintela <quintela@redhat.com>