Re: [Qemu-devel] [PATCHv2] virtio: verify that all outstanding buffers are flushed

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Dec 12, 2012 at 04:52:53PM +0100, Paolo Bonzini wrote:
> Il 12/12/2012 16:25, Michael S. Tsirkin ha scritto:
> > On Wed, Dec 12, 2012 at 04:01:27PM +0100, Paolo Bonzini wrote:
> >> Il 12/12/2012 15:47, Michael S. Tsirkin ha scritto:
> >>>> Ok, so we need some API for virtio-{blk,scsi} to communicate back the
> >>>> indexes of in-flight requests to virtio.  The indexes are known from the
> >>>> VirtQueueElement, so that's fine.
> >>>>
> >>>> Even better would be a virtio_save_request/virtio_load_request API...
> >>>
> >>> So you are saying this is a bug then? Great.
> >>
> >> I'm not sure what you mean by "it will never put the missing heads
> >> in the used ring".  The serialized requests are put in the used rings
> >> when they are completed and virtio-{blk,scsi} calls virtqueue_push.  Is
> >> the problem if you have a vring that looks like this:
> >>
> >>     A A U A U U A A
> >>
> >> ?  Which heads are leaked?  {0,1}, {2} or {6,7}?  Or a combination thereof?
> > 
> > I don't know what A A U A U U A A means.
> 
> Right, not very clear.... It means that descriptor 2 and 4 and 5 are
> free, while the others are in-flight.
> 
> > To make it work, complete all requests when vm is stopped.
> 
> That's not a choice, sorry.
> 
> >>> This is exactly what the assert above is out there to catch.
> >>> And you really can't fix it without breaking migration compatibility.
> >>
> >> Why not?  The index in the vring is in the migration data.
> > 
> > index is not enough if requests are outstanding.
> 
> Sorry, I meant the descriptor index.
> 
> > Pls check the example in the log of the patch.
> 
> I'm likewise not sure what you meant by
> 
>    A 1
>    A 2
>    U 2
>    A 2
>    U 2
>    A 2
>    U 2
>    A 2     <---
>    U 2
> 
> If I understand it, before the point marked with the arrow, the avail
> ring is
> 
>    1 2 2 2
> 
>    vring_avail_idx(vq) == 3
>    last_avail_idx == 3
> 
> After, it is
> 
>    2 2 2 2
>    vring_avail_idx(vq) == 4
>    last_avail_idx == 3
> 
> What's wrong with that?
> 
> You wrote "the only way to know head 1 is outstanding is because backend
> has stored this info somewhere".  But the backend _is_ tracking it (by
> serializing and then restoring the VirtQueueElement) and no leak happens
> because virtqueue_fill/flush will put the head on the used ring sooner
> or later.

If you did this before save vm inuse would be 0.

You said that at the point where we save state,
some entries are outstanding. It is too late to
put head at that point.


> >>> As step 1, I think we should just complete all outstanding
> >>> requests when VM stops.
> >>>
> >>> Yes it means you can't do the retry hack after migration
> >>> but this is hardly common scenario.
> >>
> >> I disagree...
> > 
> > Disagree with what? You are saying it's common?
> 
> It's not common, but you cannot block migration because you have an I/O
> error.  Solving the error may involve migrating the guests away from
> that host.
> 
> Paolo

No, you should complete with error.