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

[Alexander Duyck <alexander.duyck@gmail.com>]

On Thu, Jun 18, 2020 at 5:54 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 13.06.20 22:07, Alexander Duyck 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.
>
> Indeed, it's the name of the vmstate field. But I don't think it is
> relevant for migration. It's and indicator if a field is valid and it's
> used in traces/error messages.
>
> See git grep "field->name"
>
> I don't think renaming this is problematic. Can you rebase and resent?
> Thanks!

Okay, I will.

> > 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?
> >
> > 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
>
> The *status* should be DONE IIUC. The cmd_id might be relevant, no? It's
> always incremented until it wraps.

The thing is, the cmd_id visible to the driver if the status is DONE
is the cmd_id value for DONE. So as long as the driver acknowledges
the value we could essentially start over the cmd_id without any
negative effect. The driver would have to put down a new descriptor to
start a block of hinting in order to begin reporting again so there
shouldn't be any risk of us falsely hinting pages that were in a
previous epoch.

Ugh, although now looking at it I think we might have a bug in the
QEMU code in that the driver could in theory force its way past a
"STOP" by just replaying the last command_id descriptor and then keep
going. Should be a pretty easy fix though as we should only allow a
transition to S_START if the status is S_REQUESTED/

> > 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.
>
> We'll have to glue that to a compat machine unfortunately, so we can
> just keep migrating it ... :(

Yeah, I kind of figured that would be the case. However if the name
change is not an issue then it should not be a problem.

Thanks.

- Alex