From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: virtio-dev-return-4937-cohuck=redhat.com@lists.oasis-open.org Sender: List-Post: List-Help: List-Unsubscribe: List-Subscribe: Received: from lists.oasis-open.org (oasis-open.org [10.110.1.242]) by lists.oasis-open.org (Postfix) with ESMTP id 171029840FF for ; Wed, 7 Nov 2018 07:20:53 +0000 (UTC) From: Tiwei Bie Date: Wed, 7 Nov 2018 15:18:35 +0800 Message-Id: <20181107071835.3202-1-tiwei.bie@intel.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Subject: [virtio-dev] [PATCH] packed-ring: fix example code To: mst@redhat.com, virtio-dev@lists.oasis-open.org Cc: tiwei.bie@intel.com List-ID: 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 --- 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 --------------------------------------------------------------------- To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org