[dpdk-dev] [PATCH] virtio: fix the vq size issue

[Ouyang, Changchun]

> -----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
> >
> >
> >
> >