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

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Message-ID: <20200820151644.00e6c87c@carbon>
Date:   Thu, 20 Aug 2020 15:16:44 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Lorenzo Bianconi <lorenzo@...nel.org>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org, davem@...emloft.net,
        lorenzo.bianconi@...hat.com, echaudro@...hat.com,
        sameehj@...zon.com, kuba@...nel.org, brouer@...hat.com
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).

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?

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

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