From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Wang Subject: Re: [RFC v3 3/5] virtio_ring: add packed ring support Date: Thu, 10 May 2018 15:32:19 +0800 Message-ID: <927f4478-5a81-31d4-ac69-f9ec26248591@redhat.com> References: <20180425051550.24342-1-tiwei.bie@intel.com> <20180425051550.24342-4-tiwei.bie@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Cc: wexu@redhat.com, jfreimann@redhat.com To: Tiwei Bie , mst@redhat.com, virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org Return-path: In-Reply-To: <20180425051550.24342-4-tiwei.bie@intel.com> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On 2018年04月25日 13:15, Tiwei Bie wrote: > + /* We're using some buffers from the free list. */ > + vq->vq.num_free -= descs_used; > + > + /* Update free pointer */ > + if (indirect) { > + n = head + 1; > + if (n >= vq->vring_packed.num) { > + n = 0; > + vq->wrap_counter ^= 1; > + } > + vq->next_avail_idx = n; > + } else > + vq->next_avail_idx = i; During testing zerocopy (out of order completion), I found driver may submit two identical buffer id to vhost. So the above code may not work well. Consider the case that driver adds 3 buffer and virtqueue size is 8. a) id = 0,count = 2,next_avail = 2 b) id = 2,count = 4,next_avail = 2 c) id = 4,count = 2,next_avail = 0 if packet b is done before packet a, driver may think buffer id 0 is available and try to use it if even if the real buffer 0 was not done. Thanks