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 4:35 PM
> 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
> 
> Hi Patrick,
> 
> On 7/21/20 4:57 AM, Fu, Patrick wrote:
> > 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.
> 
> I think we should try, that would make a cleaner implementation. I don't
> think having to fallback to sync mode is a good idea because it adds an
> overhead on the CPU, which is what we try to avoid with this async mode.
> 
> Also, I am not convinced the overhead would be that significant, at least I
> hope so, otherwise it would mean this new path is just performing better
> because it takes a lot of shortcuts, like the vector path in Virtio PMD.

I can make a trial patch and do some comparison. I will try to feedback the result by this weekend.

Thanks,

Patrick