netdev - Re: [PATCH bpf-next v2 4/6] bpf, libbpf: add bpf_tail_call

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Message-ID: <55eb1fc6-f02b-ae59-768c-c17295639601@iogearbox.net>
Date:   Mon, 28 Sep 2020 21:02:48 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        john fastabend <john.fastabend@...il.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Andrii Nakryiko <andriin@...com>
Subject: Re: [PATCH bpf-next v2 4/6] bpf, libbpf: add bpf_tail_call_static
 helper for bpf programs

On 9/28/20 7:39 PM, Andrii Nakryiko wrote:
> On Mon, Sep 28, 2020 at 7:39 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>>
>> Port of tail_call_static() helper function from Cilium's BPF code base [0]
>> to libbpf, so others can easily consume it as well. We've been using this
>> in production code for some time now. The main idea is that we guarantee
>> that the kernel's BPF infrastructure and JIT (here: x86_64) can patch the
>> JITed BPF insns with direct jumps instead of having to fall back to using
>> expensive retpolines. By using inline asm, we guarantee that the compiler
>> won't merge the call from different paths with potentially different
>> content of r2/r3.
>>
>> We're also using Cilium's __throw_build_bug() macro (here as: __bpf_unreachable())
>> in different places as a neat trick to trigger compilation errors when
>> compiler does not remove code at compilation time. This works for the BPF
>> back end as it does not implement the __builtin_trap().
>>
>>    [0] https://github.com/cilium/cilium/commit/f5537c26020d5297b70936c6b7d03a1e412a1035
>>
>> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
>> Cc: Andrii Nakryiko <andriin@...com>
>> ---
> 
> few optional nits below, but looks good to me:
> 
> Acked-by: Andrii Nakryiko <andriin@...com>
[...]

Thanks!

>> +/*
>> + * Helper function to perform a tail call with a constant/immediate map slot.
>> + */
>> +static __always_inline void
>> +bpf_tail_call_static(void *ctx, const void *map, const __u32 slot)
> 
> nit: const void *ctx would work here, right? would avoid users having
> to do unnecessary casts in some cases

The bpf_tail_call() UAPI signature only has 'void *ctx' as well, so shouldn't
cause any issues as drop-in replacement wrt casts, but also ctx here is not always
a read-only [context] object either so it may potentially be a bit misleading to
users to convert it to const.