On Wed, 14 Feb 2018 01:05:16 +0530
Sunil Kovvuri <sunil.kovv...@gmail.com> wrote:

> On Tue, Feb 13, 2018 at 10:29 PM, Jesper Dangaard Brouer
> <bro...@redhat.com> wrote:
> > This reverts commit aa136d0c82fcd6af14535853c30e219e02b2692d.
> >
> > As I previously[1] pointed out this implementation of XDP_REDIRECT is
> > wrong.  XDP_REDIRECT is a facility that must work between different
> > NIC drivers.  Another NIC driver can call ndo_xdp_xmit/nicvf_xdp_xmit,
> > but your driver patch assumes payload data (at top of page) will
> > contain a queue index and a DMA addr, this is not true and worse will
> > likely contain garbage.
> >
> > Given you have not fixed this in due time (just reached v4.16-rc1),
> > the only option I see is a revert.  
> Sorry that i missed your email ie didn't respond.
> This driver is not for a generic PCI endpoint NIC, it's an on-silicon
> ethernet device found only on Cavium's ThunderX or OCTEONTX SOCs
> which supports upto 16 ports. XDP_REDIRECT here is mainly aimed at
> forwarding packets from one port to another
> port of same type.
> The only way I could have avoided the payload data is by unmapping
> and remapping of DMA buffer which is quite expensive especially when
> IOMMU is enabled. So I thought this small optimization should be
> acceptable.

It is good that you bring up the need to avoid this DMA unmap+remap
issue, but we need to solve this in a generic way, as all drivers
would/should benefit from this optimization.

> If you still think that this shouldn't be done this way then go ahead
> and revert the patch,
> I will try to redo this as and when i find time.

Yes, please revert.

In connection with AF_XDP we need to improve/adjust the ndo_xdp_xmit
API.  We/I will try to incorporate the DMA mapping avoidance into the
API design.  Then it should hopefully be easier to redo this patch later.

Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer

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