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Date: Thu, 10 May 2018 16:56:01 +0800 From: Tiwei Bie <tiwei.bie@...el.com> To: Jason Wang <jasowang@...hat.com> Cc: mst@...hat.com, virtualization@...ts.linux-foundation.org, linux-kernel@...r.kernel.org, netdev@...r.kernel.org, wexu@...hat.com Subject: Re: [RFC v3 3/5] virtio_ring: add packed ring support On Thu, May 10, 2018 at 03:34:50PM +0800, Jason Wang wrote: > 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 Nice catch! Thanks a lot! I'll implement an ID allocator. Best regards, Tiwei Bie
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