netdev - Re: [PATCH v8 bpf-next 00/14] mvneta: introduce XDP multi-buffer support

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Date:   Fri, 9 Apr 2021 22:16:14 +0200
From:   Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        ast@...nel.org, daniel@...earbox.net, shayagr@...zon.com,
        sameehj@...zon.com, dsahern@...nel.org, brouer@...hat.com,
        echaudro@...hat.com, jasowang@...hat.com,
        alexander.duyck@...il.com, saeed@...nel.org,
        maciej.fijalkowski@...el.com
Subject: Re: [PATCH v8 bpf-next 00/14] mvneta: introduce XDP multi-buffer
 support

> Lorenzo Bianconi wrote:

[...]

> 
> I just read the commit messages for v8 so far. But, I'm still wondering how
> to handle use cases where we want to put extra bytes at the end of the
> packet, or really anywhere in the general case. We can extend tail with above
> is there anyway to then write into that extra space?
> 
> I think most use cases will only want headers so we can likely make it 
> a callout to a helper. Could we add something like, xdp_get_bytes(start, end)
> to pull in the bytes?
> 
> My dumb pseudoprogram being something like,
> 
>   trailer[16] = {0,1,2,3,4,5,6,7,8,9,a,b,c,d,e}
>   trailer_size = 16;
>   old_end = xdp->length;
>   new_end = xdp->length + trailer_size;
> 
>   err = bpf_xdp_adjust_tail(xdp, trailer_size)
>   if (err) return err;
> 
>   err = xdp_get_bytes(xdp, old_end, new_end);
>   if (err) return err;
> 
>   memcpy(xdp->data, trailer, trailer_size);
> 
> Do you think that could work if we code up xdp_get_bytes()? Does the driver
> have enough context to adjust xdp to map to my get_bytes() call? I think
> so but we should check.

Hi John,

can you please give more details about how xdp_get_bytes() is expected to work?
iiuc trailer will be pulled at the beginning of the frame after updating the
xdp_buff with xdp_get_bytes helper, correct?
If so I guess it will doable, it is just a matter of reserve more space mapping
the buffer on the dma engine respect to XDP_PACKET_HEADROOM. If the frame does
not fit in a single buffer, it will be split over multiple buffers.
If you are referring to add trailer at the end of the buffer, I guess it is
doable as well introducing a bpf helper.
I guess both of the solutions are orthogonal to this series.

Regards,
Lorenzo

> 
> > 
> > More info about the main idea behind this approach can be found here [1][2].
> 
> Thanks for working on this!
> 

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