On 2018?04?25? 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
On 2018?05?10? 15:32, Jason Wang wrote:> > > On 2018?04?25? 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 = 2next_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 at lists.linux-foundation.org > https://lists.linuxfoundation.org/mailman/listinfo/virtualization
On Thu, May 10, 2018 at 03:34:50PM +0800, Jason Wang wrote:> On 2018?05?10? 15:32, Jason Wang wrote: > > On 2018?04?25? 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. > > > > ThanksNice catch! Thanks a lot! I'll implement an ID allocator. Best regards, Tiwei Bie