linux-kernel - Re: [PATCH bpf-next v2 4/8] arm32, bpf: add support for unconditional bswap instruction

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Message-ID: <ZPmUmqzvq1sV7r/f@shell.armlinux.org.uk>
Date:   Thu, 7 Sep 2023 10:15:06 +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 v2 4/8] arm32, bpf: add support for
 unconditional bswap instruction

On Thu, Sep 07, 2023 at 09:08:46AM +0000, Puranjay Mohan wrote:
> On Thu, Sep 07 2023, Russell King (Oracle) wrote:
> 
> > On Wed, Sep 06, 2023 at 06:33:16PM +0000, Puranjay Mohan wrote:
> >> @@ -1633,8 +1633,10 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx)
> >>  	/* dst = htobe(dst) */
> >>  	case BPF_ALU | BPF_END | BPF_FROM_LE:
> >>  	case BPF_ALU | BPF_END | BPF_FROM_BE:
> >> +	/* dst = bswap(dst) */
> >> +	case BPF_ALU64 | BPF_END | BPF_TO_LE:
> >>  		rd = arm_bpf_get_reg64(dst, tmp, ctx);
> >> -		if (BPF_SRC(code) == BPF_FROM_LE)
> >> +		if (BPF_SRC(code) == BPF_FROM_LE && BPF_CLASS(code) != BPF_ALU64)
> >
> > With the addition of the BPF_ALU64 case, I'm wondering why this if() is
> > affected. If you were adding:
> >
> > 	case BPF_ALU64 | BPF_END | BPF_FROM_LE:
> >
> > then maybe there would be a reason, but the BPF_ALU64 | BPF_END |
> > BPF_TO_LE case will never match even the original if() statement.
> 
> The reason is that these mean the same thing.
> from: include/uapi/linux/bpf.h
> 
> #define BPF_TO_LE	0x00	/* convert to little-endian */
> #define BPF_TO_BE	0x08	/* convert to big-endian */
> #define BPF_FROM_LE	BPF_TO_LE
> #define BPF_FROM_BE	BPF_TO_BE
> 
> So, to not cause confusion and follow the earlier cases I can add:
> 
> case BPF_ALU64 | BPF_END | BPF_FROM_LE:
> 
> in the next version.

It might be worth adding a comment after each stating one of:

	/* also BPF_TO_LE */
	/* also BPF_TO_BE */

as appropriate to make this more readable.

Thanks.

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