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Date:   Wed, 27 May 2020 09:24:20 -0600
From:   David Ahern <dsahern@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        David Ahern <dsahern@...nel.org>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        daniel@...earbox.net, john.fastabend@...il.com, ast@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com, andriin@...com
Subject: Re: [PATCH bpf-next 1/5] bpf: Handle 8-byte values in DEVMAP and
 DEVMAP_HASH

On 5/27/20 8:57 AM, Toke Høiland-Jørgensen wrote:
> 
> Either way you're creating a contract where the kernel says "first four
> bytes is the ifindex, second four bytes is the fd/id". BTF just makes
> that explicit, and allows userspace to declare that it agrees this is
> what the fields should mean. And gives us more flexibility when
> extending the API later than just adding stuff at the end and looking at
> the size...
> 
>> You need to know precisely which 4 bytes is the program fd that needs
>> to be looked up, and that AFAIK is beyond the scope of BTF.
> 
> Exactly - BTF is a way for userspace to explicitly tell the kernel "I am
> going to put the fd into these four bytes of the value field", instead
> of the kernel implicitly assuming it's always bytes 5-8.
> 

Really, I should define that struct in uapi/linux/bpf.h. The ifindex
field has special meaning: the kernel needs to convert it to a
net_device. The prog_fd field has special meaning: it should map to a
bpf program.

This use case is not in BTF's scope. But, prove me wrong. Ideas are
cheap; code is hard. Show me the code that implements your idea.

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