netdev - Re: [PATCH bpf-next 2/5] bpf: bpf_csum_diff: optimize and homogenize for all archs

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Message-ID: <b51c07f4-8987-429b-acf7-1509e649e8c0@iogearbox.net>
Date: Mon, 21 Oct 2024 15:42:23 +0200
From: Daniel Borkmann <daniel@...earbox.net>
To: Puranjay Mohan <puranjay@...nel.org>, Albert Ou <aou@...s.berkeley.edu>,
 Alexei Starovoitov <ast@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Andrii Nakryiko <andrii@...nel.org>, bpf@...r.kernel.org,
 "David S. Miller" <davem@...emloft.net>, Eduard Zingerman
 <eddyz87@...il.com>, Eric Dumazet <edumazet@...gle.com>,
 Hao Luo <haoluo@...gle.com>, Helge Deller <deller@....de>,
 Jakub Kicinski <kuba@...nel.org>,
 "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
 Jiri Olsa <jolsa@...nel.org>, John Fastabend <john.fastabend@...il.com>,
 KP Singh <kpsingh@...nel.org>, linux-kernel@...r.kernel.org,
 linux-parisc@...r.kernel.org, linux-riscv@...ts.infradead.org,
 Martin KaFai Lau <martin.lau@...ux.dev>, Mykola Lysenko <mykolal@...com>,
 netdev@...r.kernel.org, Palmer Dabbelt <palmer@...belt.com>,
 Paolo Abeni <pabeni@...hat.com>, Paul Walmsley <paul.walmsley@...ive.com>,
 Puranjay Mohan <puranjay12@...il.com>, Shuah Khan <shuah@...nel.org>,
 Song Liu <song@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
 Yonghong Song <yonghong.song@...ux.dev>
Subject: Re: [PATCH bpf-next 2/5] bpf: bpf_csum_diff: optimize and homogenize
 for all archs

On 10/21/24 2:21 PM, Puranjay Mohan wrote:
> 1. Optimization
>     ------------
> 
> The current implementation copies the 'from' and 'to' buffers to a
> scratchpad and it takes the bitwise NOT of 'from' buffer while copying.
> In the next step csum_partial() is called with this scratchpad.
> 
> so, mathematically, the current implementation is doing:
> 
> 	result = csum(to - from)
> 
> Here, 'to'  and '~ from' are copied in to the scratchpad buffer, we need
> it in the scratchpad buffer because csum_partial() takes a single
> contiguous buffer and not two disjoint buffers like 'to' and 'from'.
> 
> We can re write this equation to:
> 
> 	result = csum(to) - csum(from)
> 
> using the distributive property of csum().
> 
> this allows 'to' and 'from' to be at different locations and therefore
> this scratchpad and copying is not needed.
> 
> This in C code will look like:
> 
> result = csum_sub(csum_partial(to, to_size, seed),
>                    csum_partial(from, from_size, 0));
> 
> 2. Homogenization
>     --------------
> 
> The bpf_csum_diff() helper calls csum_partial() which is implemented by
> some architectures like arm and x86 but other architectures rely on the
> generic implementation in lib/checksum.c
> 
> The generic implementation in lib/checksum.c returns a 16 bit value but
> the arch specific implementations can return more than 16 bits, this
> works out in most places because before the result is used, it is passed
> through csum_fold() that turns it into a 16-bit value.
> 
> bpf_csum_diff() directly returns the value from csum_partial() and
> therefore the returned values could be different on different
> architectures. see discussion in [1]:
> 
> for the int value 28 the calculated checksums are:
> 
> x86                    :    -29 : 0xffffffe3
> generic (arm64, riscv) :  65507 : 0x0000ffe3
> arm                    : 131042 : 0x0001ffe2
> 
> Pass the result of bpf_csum_diff() through from32to16() before returning
> to homogenize this result for all architectures.
> 
> NOTE: from32to16() is used instead of csum_fold() because csum_fold()
> does from32to16() + bitwise NOT of the result, which is not what we want
> to do here.
> 
> [1] https://lore.kernel.org/bpf/CAJ+HfNiQbOcqCLxFUP2FMm5QrLXUUaj852Fxe3hn_2JNiucn6g@mail.gmail.com/
> 
> Signed-off-by: Puranjay Mohan <puranjay@...nel.org>

Thanks for looking into this!

Acked-by: Daniel Borkmann <daniel@...earbox.net>