Re: [dpdk-dev] [PATCH v1] vhost: support cross page buf in async data path

["Fu, Patrick" <patrick.fu@intel.com>]

Hi Maxime,

> -----Original Message-----
> From: Maxime Coquelin <maxime.coquelin@redhat.com>
> Sent: Tuesday, July 21, 2020 12:40 AM
> To: Fu, Patrick <patrick.fu@intel.com>; dev@dpdk.org; Xia, Chenbo
> <chenbo.xia@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.fu@intel.com wrote:
> > From: Patrick Fu <patrick.fu@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.fu@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);
> >