netdev - Re: [RFC bpf-next 2/6] net: xdp: RX meta data infrastructure

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Date:   Wed, 4 Jul 2018 09:51:54 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Saeed Mahameed <saeedm@...lanox.com>,
        "alexei.starovoitov@...il.com" <alexei.starovoitov@...il.com>,
        "saeedm@....mellanox.co.il" <saeedm@....mellanox.co.il>
Cc:     "alexander.h.duyck@...el.com" <alexander.h.duyck@...el.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "brouer@...hat.com" <brouer@...hat.com>,
        "borkmann@...earbox.net" <borkmann@...earbox.net>,
        "peter.waskiewicz.jr@...el.com" <peter.waskiewicz.jr@...el.com>
Subject: Re: [RFC bpf-next 2/6] net: xdp: RX meta data infrastructure

On 07/04/2018 02:57 AM, Saeed Mahameed wrote:
> On Tue, 2018-07-03 at 16:01 -0700, Alexei Starovoitov wrote:
[...]
>> How about we make driver+firmware provide a BTF definition of
>> metadata that they
>> can provide? There can be multiple definitions of such structs.
>> Then in userpsace we can have BTF->plain C converter.
>> (bpftool practically ready to do that already).
>> Then the programmer can take such generated C definition, add it to
>> .h and include
>> it in their programs. llvm will compile the whole thing and will
>> include BTF
>> of maps, progs and this md struct in the target elf file.
>> During loading the kernel can check that BTF in elf is matching one-
>> to-one
>> to what driver+firmware are saying they support.

I do like the above idea of utilizing BTF for this, seems like a good fit.

> Just thinking out loud, can't we do this at program load ? just run a
> setup function in the xdp program to load nic md BTF definition into
> the elf section ?
> 
>> No ambiguity and no possibility of mistake, since offsets and field
>> names
>> are verified.
> 
> But what about the dynamic nature of this feature ? Sometimes you only
> want HW/Driver to provide a subset of whatever the HW can provide and
> save md buffer for other stuff.
> 
> Yes a well defined format is favorable here, but we need to make sure
> there is no computational overhead in data path just to extract each
> field! for example if i want to know what is the offset of the hash
> will i need to go parse (for every packet) the whole BTF definition of
> metadata just to find the offset of type=hash ?

I don't think this would be the case that you'd need to walk BTF in fast
path here. In the ideal case, the only thing that driver would need to do
in fast path would be to set proper xdp->data_meta offset and _that_ would
be it. For the rest, program would know how to access the data since it's
already aware of it from BTF definition the driver provided. Other drivers
which would be less flexible on that regard would internally prep the buffer
based on the progs needs more or less similar as in mlx5e_xdp_fill_data_meta(),
but it would be really up to the driver how to handle this internally. The
BTF it would check at XDP setup time to do the configuration needed in the
driver. Verifier would only check BTF, pass it along for XDP setup, prog
rewrites in verifier aren't even needed since LLVM compiled everything
already.

>> Every driver can have their own BTF for md and their own special
>> features.
>> We can try to standardize the names (like vlan and csum), so xdp
>> programs
>> can stay relatively portable across NICs.
> 
> Yes this is a must.

Agree, there needs to be a basic common set that would be provided by
every XDP aware driver.

>> Such api will address exposing asic+firmware metadata to the xdp
>> program.
>> Once we tackle this problem, we'll think how to do the backward
>> config
>> (to do firmware reconfig for specific BTF definition of md supplied
>> by the prog).
>> What people think?
> 
> For legacy HW, we can do it already in the driver, provide whatever the
> prog requested, its only a matter of translation to the BTF format in
> the driver xdp setup and pushing the values accordingly into the md
> offsets on data path.
> 
> Question: how can you share the md BTF from the driver/HW with the xdp
> program ?
I think this would likely be a new query as in XDP_QUERY_META_BTF
implemented in ndo_bpf callback and then exported e.g. via bpf(2)
or netlink such that bpftool can generate BTF -> C from there for the
program to include later in compilation.

Thanks,
Daniel