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Subject: [PATCH] packed-ring: fix example code
We also need to check whether AVAIL bit equals to USED bit when checking whether a descriptor is a used descriptor. 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 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 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 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 device made that descriptor used, the ring will be: +-----------+----+----+----+ | 00 (id=0) | 10 | 11 | 10 | +-----------+----+----+----+ If driver doesn't check whether AVAIL bit equals to USED bit when checking whether a descriptor is a used descriptor, after checking the first descriptor (whose AVAIL and USED bits are 0 and 0) and advancing the 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) as used which is wrong. Signed-off-by: Tiwei Bie <tiwei.bie@intel.com> --- packed-ring.tex | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/packed-ring.tex b/packed-ring.tex index ebbad95..ec116d7 100644 --- a/packed-ring.tex +++ b/packed-ring.tex @@ -688,15 +688,17 @@ for (;;) { struct pvirtq_desc *d = vq->desc[vq->next_used]; flags = d->flags; + bool avail = flags & VIRTQ_DESC_F_AVAIL; bool used = flags & VIRTQ_DESC_F_USED; - if (used != vq->used_wrap_count) { + if (avail != used || used != vq->used_wrap_count) { vq->driver_event.flags = RING_EVENT_FLAGS_ENABLE; memory_barrier(); flags = d->flags; + bool avail = flags & VIRTQ_DESC_F_AVAIL; bool used = flags & VIRTQ_DESC_F_USED; - if (used != vq->used_wrap_count) { + if (avail != used || used != vq->used_wrap_count) { break; } -- 2.19.1
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