From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934641AbeEJHe6 (ORCPT ); Thu, 10 May 2018 03:34:58 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:42168 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S933589AbeEJHe5 (ORCPT ); Thu, 10 May 2018 03:34:57 -0400 Subject: Re: [RFC v3 3/5] virtio_ring: add packed ring support From: Jason Wang To: Tiwei Bie , mst@redhat.com, virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org Cc: wexu@redhat.com References: <20180425051550.24342-1-tiwei.bie@intel.com> <20180425051550.24342-4-tiwei.bie@intel.com> <927f4478-5a81-31d4-ac69-f9ec26248591@redhat.com> Message-ID: <5885acac-e9e3-3abf-b6a2-7347f4d55be2@redhat.com> Date: Thu, 10 May 2018 15:34:50 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <927f4478-5a81-31d4-ac69-f9ec26248591@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2018年05月10日 15:32, Jason Wang wrote: > > > 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 next_avail should be 6 here. > > c) id = 4,count = 2,next_avail = 0 > id should be 6 here. Thanks > 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 > _______________________________________________ > Virtualization mailing list > Virtualization@lists.linux-foundation.org > https://lists.linuxfoundation.org/mailman/listinfo/virtualization