linux-kernel - Re: [PATCH bpf-next 4/8] arm32, bpf: add support for unconditional bswap instruction

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Message-ID: <ZPeecV807AVEkCJB@shell.armlinux.org.uk>
Date:   Tue, 5 Sep 2023 22:32:33 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Puranjay Mohan <puranjay12@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>,
        Yonghong Song <yonghong.song@...ux.dev>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Shubham Bansal <illusionist.neo@...il.com>,
        Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
        bpf@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next 4/8] arm32, bpf: add support for unconditional
 bswap instruction

On Tue, Sep 05, 2023 at 09:06:17PM +0000, Puranjay Mohan wrote:
> The cpuv4 added a new unconditional bswap instruction with following
> behaviour:
> 
> BPF_ALU64 | BPF_TO_LE | BPF_END with imm = 16/32/64 means:
> dst = bswap16(dst)
> dst = bswap32(dst)
> dst = bswap64(dst)
> 
> As we already support converting to big-endian from little-endian we can
> use the same for unconditional bswap.
> Since ARM32 is always little-endian, just treat the unconditional scenario

This is not true. Arm32 BPF is disabled for BE32 but not for BE8. It's
entirely possible to build a kernel using BE8 for ARMv7 and have the
BPF JIT enabled:

        select HAVE_EBPF_JIT if !CPU_ENDIAN_BE32

So it's not true to say "Since ARM32 is always little-endian".

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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