> -----Original Message----- > From: Xie, Huawei > Sent: Thursday, July 2, 2015 10:02 AM > To: Ouyang, Changchun; dev at dpdk.org; Thomas Monjalon > Subject: Re: [dpdk-dev] [PATCH] virtio: fix the vq size issue > > On 7/2/2015 8:29 AM, Ouyang, Changchun wrote: > > Hi huawei, > > > >> -----Original Message----- > >> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Xie, Huawei > >> Sent: Wednesday, July 1, 2015 11:53 PM > >> To: dev at dpdk.org; Thomas Monjalon > >> Subject: Re: [dpdk-dev] [PATCH] virtio: fix the vq size issue > >> > >> On 7/1/2015 3:49 PM, Ouyang Changchun wrote: > >>> This commit breaks virtio basic packets rx functionality: > >>> d78deadae4dca240e85054bf2d604a801676becc > >>> > >>> The QEMU use 256 as default vring size, also use this default value > >>> to calculate the virtio avail ring base address and used ring base > >>> address, and vhost in the backend use the ring base address to do > >>> packet > >> IO. > >>> Virtio spec also says the queue size in PCI configuration is > >>> read-only, so virtio front end can't change it. just need use the > >>> read-only value to allocate space for vring and calculate the avail > >>> and used ring base address. Otherwise, the avail and used ring base > >> address will be different between host and guest, accordingly, packet > >> IO can't work normally. > >> virtio driver could still use the vq_size to initialize avail ring > >> and use ring so that they still have the same base address. > >> The other issue is vhost use index & (vq->size -1) to index the ring. > > I am not sure what is your clear message here, Vhost has no choice but > > use vq->size -1 to index the ring, It is qemu that always use 256 as > > the vq size, and set the avail and used ring base address, It also > > tells vhost the vq size is 256. > > I mean "the same base address issue" could be resolved, but we still couldn't > stop vhost using idx & vq->size -1 to index the ring. >
Then this patch will resolve this avail ring base address issue. > >> > >> Thomas: > >> This fix works but introduces slight change with original code. Could > >> we just rollback that commit? > > What's your major concern for the slight change here? > > just removing the unnecessary check for nb_desc itself. > > So I think no issue for the slight change. > > No major concern. It is better if this patch just rollbacks that commit > without > introduce extra change if not necessary. > The original code set nb_desc to vq_size, though it isn't used later. > I prefer to have the slight change to remove unnecessary setting. > > > > Thanks > > Changchun > > > > > > > >
