netdev - Re: [PATCH net-next 0/6] mvneta: introduce XDP multi-buffer support

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Date:   Thu, 20 Aug 2020 15:36:29 +0200
From:   Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        BPF-dev-list <bpf@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eelco Chaudron <echaudro@...hat.com>,
        "Jubran, Samih" <sameehj@...zon.com>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH net-next 0/6] mvneta: introduce XDP multi-buffer support

>
>
> General issue (that I think must be resolved/discussed as part of this initial
> patchset).

I was thinking about this issue as well.

>
> When XDP_REDIRECT'ing a multi-buffer xdp_frame out of another driver's
> ndo_xdp_xmit(), what happens if the remote driver doesn't understand the
> multi-buffer format?
>
> My guess it that it will only send the first part of the packet (in the
> main page). Fortunately we don't leak memory, because xdp_return_frame()
> handle freeing the other segments. I assume this isn't acceptable
> behavior... or maybe it is?
>
> What are our options for handling this:
>
> 1. Add mb support in ndo_xdp_xmit in every driver?

I guess this is the optimal approach.

>
> 2. Drop xdp->mb frames inside ndo_xdp_xmit (in every driver without support)?

Probably this is the easiest solution.
Anyway if we drop patch 6/6 this is not a real issue since the driver
is not allowed yet to receive frames bigger than one page and we have
time to address this issue in each driver.

Regards,
Lorenzo

>
> 3. Add core-code check before calling ndo_xdp_xmit()?
>
> --Jesper
>
> On Wed, 19 Aug 2020 15:13:45 +0200 Lorenzo Bianconi <lorenzo@...nel.org> wrote:
>
> > Finalize XDP multi-buffer support for mvneta driver introducing the capability
> > to map non-linear buffers on tx side.
> > Introduce multi-buffer bit (mb) in xdp_frame/xdp_buffer to specify if
> > shared_info area has been properly initialized.
> > Initialize multi-buffer bit (mb) to 0 in all XDP-capable drivers.
> > Add multi-buff support to xdp_return_{buff/frame} utility routines.
> >
> > Changes since RFC:
> > - squash multi-buffer bit initialization in a single patch
> > - add mvneta non-linear XDP buff support for tx side
>
> --
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: http://www.linkedin.com/in/brouer
>