* Halil Pasic (pa...@linux.vnet.ibm.com) wrote:
> We have a migration problem, which is in my opinion caused by a
> deficiency in how vq->inuse is calculated after the migration (commit
> bccdef6b "virtio: recalculate vq->inuse after migration" to
> blame).
>
>
> We got a bugreport with this log for a live migration target.
>
> 2016-12-13T18:59:03.647309Z qemu-system-s390x: VQ 1 size 0x100 <
> last_avail_idx 0x2f76 - used_idx 0x762f
Is this actually an endian problem - 2f76 vs 762f ?
Dave
> 2016-12-13T18:59:03.647385Z qemu-system-s390x: error while loading state for
> instance 0x0 of device '/fe.0.0001/virtio-net'
> 2016-12-13T18:59:03.647540Z qemu-system-s390x: load of migration failed:
> Operation not permitted
> 2016-12-13 18:59:03.796+0000: shutting down, reason=failed
>
> They use QEMU version 2.7 but looking at the current git master
> I think this did not get fixed in the meanwhile.
>
> So here goes the argument. The recalculation is done like this:
>
> + vdev->vq[i].inuse = vdev->vq[i].last_avail_idx -
> + vdev->vq[i].used_idx;
>
> This does not seem correct when last_avail_idx has already
> wrapped around but used_idx not yet. We see from the log that
> last_avail_idx (0x2f76) less that used_idx (0x762f) thus
> inuse (of type int) ends up being negative.
>
> + if (vdev->vq[i].inuse > vdev->vq[i].vring.num) {
>
> Because vdev->vq[i].vring.num is unsigned int ala usual arithmetic
> conversions ("Otherwise, if the operand that has unsigned integer type
> has rank greater or equal to the rank of the type of the other operand,
> then the operand with signed integer type is converted to the type of
> the operand with unsigned integer type." C99) inuse gets converted to
> unsigned int.
>
> Thus the check fails and produces the log cited above.
>
> + error_report("VQ %d size 0x%x < last_avail_idx 0x%x - "
> + "used_idx 0x%x",
> + i, vdev->vq[i].vring.num,
> + vdev->vq[i].last_avail_idx,
> + vdev->vq[i].used_idx);
> + return -1;
> + }
>
> Do we want to try to fix this for 2.8? I already have a small patch prepared.
>
> Regards,
> Halil
>
>
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
