netdev - Re: [PATCH bpf-next 0/5] AF_XDP: bug fixes and descriptor changes

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Date:   Mon, 4 Jun 2018 09:24:31 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     magnus.karlsson@...el.com, magnus.karlsson@...il.com,
        alexander.h.duyck@...el.com, alexander.duyck@...il.com, ast@...com,
        brouer@...hat.com, daniel@...earbox.net, netdev@...r.kernel.org,
        mykyta.iziumtsev@...aro.org,
        Björn Töpel <bjorn.topel@...el.com>,
        john.fastabend@...il.com, willemdebruijn.kernel@...il.com,
        mst@...hat.com, michael.lundkvist@...csson.com,
        jesse.brandeburg@...el.com, anjali.singhai@...el.com,
        qi.z.zhang@...el.com, francois.ozog@...aro.org,
        ilias.apalodimas@...aro.org, brian.brooks@...aro.org,
        andy@...yhouse.net, michael.chan@...adcom.com
Subject: Re: [PATCH bpf-next 0/5] AF_XDP: bug fixes and descriptor changes

On Mon, Jun 04, 2018 at 01:57:10PM +0200, Björn Töpel wrote:
> From: Björn Töpel <bjorn.topel@...el.com>
> 
> An issue with the current AF_XDP uapi raised by Mykyta Iziumtsev (see
> https://www.spinics.net/lists/netdev/msg503664.html) is that it does
> not support NICs that have a "type-writer" model in an efficient
> way. In this model, a memory window is passed to the hardware and
> multiple frames might be filled into that window, instead of just one
> that we have in the current fixed frame-size model.
> 
> This patch set fixes two bugs in the current implementation and then
> changes the uapi so that the type-writer model can be supported
> efficiently by a possible future extension of AF_XDP.
> 
> These are the uapi changes in this patch:
> 
> * Change the "u32 idx" in the descriptors to "u64 addr". The current
>   idx based format does NOT work for the type-writer model (as packets
>   can start anywhere within a frame) but that a relative address
>   pointer (the u64 addr) works well for both models in the prototype
>   code we have that supports both models. We increased it from u32 to
>   u64 to support umems larger than 4G. We have also removed the u16
>   offset when having a "u64 addr" since that information is already
>   carried in the least significant bits of the address.
> 
> * We want to use "u8 padding[5]" for something useful in the future
>   (since we are not allowed to change its name), so we now call it
>   just options so it can be extended for various purposes in the
>   future. It is an u32 as that it what is left of the 16 byte
>   descriptor.
> 
> * We changed the name of frame_size in the UMEM_REG setsockopt to
>   chunk_size since this naming also makes sense to the type-writer
>   model.
> 
> With these changes to the uapi, we believe the type-writer model can
> be supported without having to resort to a new descriptor format. The
> type-writer model could then be supported, from the uapi point of
> view, by setting a flag at bind time and providing a new flag bit in
> the options field of the descriptor that signals to user space that
> all packets have been written in a chunk. Or with a new chunk
> completion queue as suggested by Mykyta in his latest feedback mail on
> the list.

for the set:
Acked-by: Alexei Starovoitov <ast@...nel.org>
Thank you for these fixes.
According to unofficial feedback from brcm and netronome folks
the descriptor format should work for these nics too.
At some point we may consider second format, but I think SW
should drive HW requirements and not the other way around.