Hi Maxime,
> -----Original Message-----
> From: Maxime Coquelin <maxime.coque...@redhat.com>
> Sent: Tuesday, July 21, 2020 12:40 AM
> To: Fu, Patrick <patrick...@intel.com>; dev@dpdk.org; Xia, Chenbo
> <chenbo....@intel.com>
> Subject: Re: [PATCH v1] vhost: support cross page buf in async data path
>
> The title could be improved, it is not very clear IMHO.
How about:
vhost: fix async copy failure on buffers cross page boundary
> On 7/20/20 4:52 AM, patrick...@intel.com wrote:
> > From: Patrick Fu <patrick...@intel.com>
> >
> > Async copy fails when ring buffer cross two physical pages. This patch
> > fix the failure by letting copies occur in sync mode if crossing page
> > buffers are given.
>
> Wouldn't it be possible to have the buffer split into two iovecs?
Technically we can do that, however, it will also introduce significant
overhead:
- overhead from adding additional logic in vhost async data path to handle the
case
- overhead from dma device to consume 2 iovecs
In average, I don't think dma copy can benefit too much for the buffer which
are split into multiple pages.
CPU copy shall be a more suitable method.
> > Fixes: cd6760da1076 ("vhost: introduce async enqueue for split ring")
> >
> > Signed-off-by: Patrick Fu <patrick...@intel.com>
> > ---
> > lib/librte_vhost/virtio_net.c | 12 +++---------
> > 1 file changed, 3 insertions(+), 9 deletions(-)
> >
> > diff --git a/lib/librte_vhost/virtio_net.c
> > b/lib/librte_vhost/virtio_net.c index 1d0be3dd4..44b22a8ad 100644
> > --- a/lib/librte_vhost/virtio_net.c
> > +++ b/lib/librte_vhost/virtio_net.c
> > @@ -1071,16 +1071,10 @@ async_mbuf_to_desc(struct virtio_net *dev,
> struct vhost_virtqueue *vq,
> > }
> >
> > cpy_len = RTE_MIN(buf_avail, mbuf_avail);
> > + hpa = (void *)(uintptr_t)gpa_to_hpa(dev,
> > + buf_iova + buf_offset, cpy_len);
> >
> > - if (unlikely(cpy_len >= cpy_threshold)) {
> > - hpa = (void *)(uintptr_t)gpa_to_hpa(dev,
> > - buf_iova + buf_offset, cpy_len);
> > -
> > - if (unlikely(!hpa)) {
> > - error = -1;
> > - goto out;
> > - }
> > -
> > + if (unlikely(cpy_len >= cpy_threshold && hpa)) {
> > async_fill_vec(src_iovec + tvec_idx,
> > (void *)(uintptr_t)rte_pktmbuf_iova_offset(m,
> > mbuf_offset), cpy_len);
> >
