netdev - Re: [net PATCH] Revert "net: thunderx: Add support for xdp redirect"

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Message-ID: <20180214094915.10fc6c36@redhat.com>
Date:   Wed, 14 Feb 2018 09:49:15 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Sunil Kovvuri <sunil.kovvuri@...il.com>
Cc:     Linux Netdev List <netdev@...r.kernel.org>,
        Sunil Goutham <sgoutham@...ium.com>,
        Aleksey Makarov <aleksey.makarov@...ium.com>,
        "David S. Miller" <davem@...emloft.net>,
        Christina Jacob <cjacob@...iumnetworks.com>,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        LAKML <linux-arm-kernel@...ts.infradead.org>, brouer@...hat.com,
        Björn Töpel 
        <bjorn.topel@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>
Subject: Re: [net PATCH] Revert "net: thunderx: Add support for xdp
 redirect"

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

> On Tue, Feb 13, 2018 at 10:29 PM, Jesper Dangaard Brouer
> <brouer@...hat.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