Re: [dpdk-dev] [PATCH] net/virtio: enable packet data prefetch on x86

["Liu, Yong" <yong.liu@intel.com>]

> -----Original Message-----
> From: David Marchand <david.marchand@redhat.com>
> Sent: Thursday, November 12, 2020 4:58 PM
> To: Liu, Yong <yong.liu@intel.com>; Maxime Coquelin
> <maxime.coquelin@redhat.com>
> Cc: Xia, Chenbo <chenbo.xia@intel.com>; dev <dev@dpdk.org>; Richardson,
> Bruce <bruce.richardson@intel.com>
> Subject: Re: [dpdk-dev] [PATCH] net/virtio: enable packet data prefetch on
> x86
> 
> On Thu, Nov 12, 2020 at 9:48 AM Maxime Coquelin
> <maxime.coquelin@redhat.com> wrote:
> > On 11/11/20 4:40 PM, Marvin Liu wrote:
> > > Data prefetch instruction can preload data into cpu’s hierarchical
> > > cache before data access. Virtio datapath utilized this feature for
> > > data access acceleration. As config RTE_PMD_PACKET_PREFETCH was
> > > discarded, now packet data prefetch is enabled based on architecture.
> > >
> > > Signed-off-by: Marvin Liu <yong.liu@intel.com>
> > >
> > > diff --git a/drivers/net/virtio/virtqueue.h
> b/drivers/net/virtio/virtqueue.h
> > > index 42c4c9882..0196290a5 100644
> > > --- a/drivers/net/virtio/virtqueue.h
> > > +++ b/drivers/net/virtio/virtqueue.h
> > > @@ -106,7 +106,7 @@ virtqueue_store_flags_packed(struct
> vring_packed_desc *dp,
> > >               dp->flags = flags;
> > >       }
> > >  }
> > > -#ifdef RTE_PMD_PACKET_PREFETCH
> > > +#if defined(RTE_ARCH_X86)
> > >  #define rte_packet_prefetch(p)  rte_prefetch1(p)
> > >  #else
> > >  #define rte_packet_prefetch(p)  do {} while(0)
> > >
> >
> > Thanks for catching this issue.
> > I agree it should be re-enabled by default, and not only on X86, not
> > only on Virtio PMD.
> >
> > AFAICS, prefetch was enabled for all platforms before the switch to
> > Meson, so I see it as an involuntary change that needs to be reverted.
> 
> Yes and this will also solve https://patchwork.dpdk.org/patch/83468/.
> Thanks.
> 

Agreed, original patch was intended to recover prefetch configuration in meson build. 
Please check http://patchwork.dpdk.org/patch/78451/. 
And it leaded a discussion about how to utilize prefetch function optimally. 
Due to no conclusion for current position is best for other platforms except x86, now only enable prefetch in virtio + x86. 

> --
> David Marchand