netdev - Re: [PATCH v9 bpf-next 02/14] xdp: introduce flags field in xdp_buff/xdp

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Date:   Mon, 5 Jul 2021 17:52:49 +0200
From:   Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf <bpf@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, shayagr@...zon.com,
        "Jubran, Samih" <sameehj@...zon.com>,
        John Fastabend <john.fastabend@...il.com>,
        David Ahern <dsahern@...nel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Saeed Mahameed <saeed@...nel.org>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Tirthendu <tirthendu.sarkar@...el.com>
Subject: Re: [PATCH v9 bpf-next 02/14] xdp: introduce flags field in
 xdp_buff/xdp_frame

> On Tue, Jun 29, 2021 at 5:43 AM Lorenzo Bianconi
> <lorenzo.bianconi@...hat.com> wrote:
> >
> > > On Mon, Jun 14, 2021 at 5:50 AM Lorenzo Bianconi <lorenzo@...nel.org> wrote:
> > > >
> > > > Introduce flags field in xdp_frame/xdp_buffer data structure
> > > > to define additional buffer features. At the moment the only
> > > > supported buffer feature is multi-buffer bit (mb). Multi-buffer bit
> > > > is used to specify if this is a linear buffer (mb = 0) or a multi-buffer
> > > > frame (mb = 1). In the latter case the shared_info area at the end of
> > > > the first buffer will be properly initialized to link together
> > > > subsequent buffers.
> > > >
> > > > Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
> > >
> > > Instead of passing this between buffers and frames I wonder if this
> > > wouldn't be better to place in something like the xdp_mem_info
> > > structure since this is something that would be specific to how the
> > > device is handling memory anyway. You could probably split the type
> > > field into a 16b type and a 16b flags field. Then add your bit where 0
> > > is linear/legacy and 1 is scatter-gather/multi-buffer.
> > >
> >
> > ack, this should be fine but I put the flag field in xdp_buff/xdp_frame
> > in order to reuse it for some xdp hw-hints (e.g rx checksum type).
> > We can put it in xdp_mem_info too but I guess it would be less intuitive, what
> > do you think?
> 
> I think it makes the most sense in xdp_mem_info. It already tells us
> what to expect in some respect in regards to memory layout as it tells
> us if we are dealing with shared pages or whole pages and how to
> recycle them. I would think that applies almost identically to
> scatter-gather XDP the same way.

Hi Alex,

Reviewing the code to address this comment I think I spotted a corner case
where we can't use this approach. Whenever we run dev_map_bpf_prog_run()
we loose mb info converting xdp_frame to xdp_buff since
xdp_convert_frame_to_buff() does not copy it and we have no xdp_rxq_info there.
Do you think we should add a rxq_info there similar to what we did for cpumap?
I think it is better to keep the previous approach since it seems cleaner and
reusable in the future. What do you think?

Regards,
Lorenzo

> 
> As far as the addition of flags there is still time for that later as
> we still have the 32b of unused space after frame_sz.
> 

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