Re: [virtio-dev] Re: [PATCH v25 QEMU 3/3] virtio-balloon: Replace free page hinting references to 'report' with 'hint'

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

* Alexander Duyck (alexander.duyck@gmail.com) wrote:
> On Tue, May 26, 2020 at 9:14 PM Alexander Duyck
> <alexander.duyck@gmail.com> wrote:
> >
> > From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> >
> > In an upcoming patch a feature named Free Page Reporting is about to be
> > added. In order to avoid any confusion we should drop the use of the word
> > 'report' when referring to Free Page Hinting. So what this patch does is go
> > through and replace all instances of 'report' with 'hint" when we are
> > referring to free page hinting.
> >
> > Acked-by: David Hildenbrand <david@redhat.com>
> > Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> > ---
> >  hw/virtio/virtio-balloon.c         |   78 ++++++++++++++++++------------------
> >  include/hw/virtio/virtio-balloon.h |   20 +++++----
> >  2 files changed, 49 insertions(+), 49 deletions(-)
> >
> > diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> > index 3e2ac1104b5f..dc15409b0bb6 100644
> > --- a/hw/virtio/virtio-balloon.c
> > +++ b/hw/virtio/virtio-balloon.c
> 
> ...
> 
> > @@ -817,14 +817,14 @@ static int virtio_balloon_post_load_device(void *opaque, int version_id)
> >      return 0;
> >  }
> >
> > -static const VMStateDescription vmstate_virtio_balloon_free_page_report = {
> > +static const VMStateDescription vmstate_virtio_balloon_free_page_hint = {
> >      .name = "virtio-balloon-device/free-page-report",
> >      .version_id = 1,
> >      .minimum_version_id = 1,
> >      .needed = virtio_balloon_free_page_support,
> >      .fields = (VMStateField[]) {
> > -        VMSTATE_UINT32(free_page_report_cmd_id, VirtIOBalloon),
> > -        VMSTATE_UINT32(free_page_report_status, VirtIOBalloon),
> > +        VMSTATE_UINT32(free_page_hint_cmd_id, VirtIOBalloon),
> > +        VMSTATE_UINT32(free_page_hint_status, VirtIOBalloon),
> >          VMSTATE_END_OF_LIST()
> >      }
> >  };
> 
> So I noticed this patch wasn't in the list of patches pulled, but that
> is probably for the best since I believe the change above might have
> broken migration as VMSTATE_UINT32 does a stringify on the first
> parameter.
> Any advice on how to address it, or should I just give up on renaming
> free_page_report_cmd_id and free_page_report_status?

The filed names never hit the wire; the migration format is trivial
binary, especially of things like integers - that lands as just 4 bytes
on the wire [ hopefully in the place where the destination expects to
receive them ].
You need to be careful of the names of top level vmstate devices, and
the names of subsections; I don't think any other naming is in the
stream.
(We've even done hacks in the past of converting a VMSTATE_UINT32 to a
pair of UINT16 )

Dave

> Looking at this I wonder why we even need to migrate these values? It
> seems like if we are completing a migration the cmd_id should always
> be "DONE" shouldn't it? It isn't as if we are going to migrate the
> hinting from one host to another. We will have to start over which is
> essentially the signal that the "DONE" value provides. Same thing for
> the status. We shouldn't be able to migrate unless both of these are
> already in the "DONE" state so if anything I wonder if we shouldn't
> have that as the initial state for the device and just drop the
> migration info.
> 
> Thanks.
> 
> - Alex
> 
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--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK