packed-ring: fix used descriptor checking in example code

When the driver is processing used descriptors in parallel
with adding new available descriptors, the driver can't just
check whether USED bit equals to the used wrap counter when
checking whether a descriptor is a used descriptor, because
the driver also needs to check whether the descriptor has
been made available. Below is an example:

Assuming ring size is 4, ring's initial state will be:

+----+----+----+----+
| 00 | 00 | 00 | 00 |
+----+----+----+----+

00 means AVAIL=0 USED=0, 01 means AVAIL=0 USED=1
10 means AVAIL=1 USED=0, 11 means AVAIL=1 USED=1

After the driver made two descriptor chains available and
each chain consists of two descriptors, the ring could be:

+----+-----------+----+-----------+
| 10 | 10 (id=0) | 10 | 10 (id=1) |
+----+-----------+----+-----------+

After the device processed all the available descriptors and
made them used (e.g. in order), the ring could be:

+-----------+----+-----------+----+
| 11 (id=0) | 10 | 11 (id=1) | 10 |
+-----------+----+-----------+----+

After the driver processed all the used descriptors and made
one descriptor (not chained, just one descriptor) available,
the ring could be:

+-----------+----+----+----+
| 01 (id=0) | 10 | 11 | 10 |
+-----------+----+----+----+

After the device made that descriptor used, the ring will be:

+-----------+----+----+----+
| 00 (id=0) | 10 | 11 | 10 |
+-----------+----+----+----+

If the driver just checks whether USED bit equals to the used
wrap counter when checking whether a descriptor is a used
descriptor, after processing the first descriptor (whose
AVAIL and USED bits are both 0), and advancing vq->next_used
pointer, it will then also treat the next descriptor, i.e.
the second descriptor (whose AVAIL and USED bits are 1 and
0 respectively) as a used descriptor which is wrong.

Fixes: https://github.com/oasis-tcs/virtio-spec/issues/29
Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Approved-by: https://www.oasis-open.org/committees/ballot.php?id=3184

0 files changed, 0 insertions, 0 deletions