linux-kernel - Re: [PATCH bpf-next v6 2/4] arm64: Add LDR (literal) instruction

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Date:   Tue, 5 Jul 2022 17:39:41 +0100
From:   Will Deacon <will@...nel.org>
To:     Xu Kuohai <xukuohai@...wei.com>
Cc:     bpf@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Zi Shen Lim <zlim.lnx@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Russell King <rmk+kernel@...linux.org.uk>,
        James Morse <james.morse@....com>,
        Hou Tao <houtao1@...wei.com>,
        Jason Wang <wangborong@...rlc.com>
Subject: Re: [PATCH bpf-next v6 2/4] arm64: Add LDR (literal) instruction

On Sat, Jun 25, 2022 at 12:12:53PM -0400, Xu Kuohai wrote:
> Add LDR (literal) instruction to load data from address relative to PC.
> This instruction will be used to implement long jump from bpf prog to
> bpf rampoline in the follow-up patch.

typo: trampoline

> 
> The instruction encoding:
> 
>     3       2   2     2                                     0        0
>     0       7   6     4                                     5        0
> +-----+-------+---+-----+-------------------------------------+--------+
> | 0 x | 0 1 1 | 0 | 0 0 |                imm19                |   Rt   |
> +-----+-------+---+-----+-------------------------------------+--------+
> 
> for 32-bit, variant x == 0; for 64-bit, x == 1.
> 
> branch_imm_common() is used to check the distance between pc and target
> address, since it's reused by this patch and LDR (literal) is not a branch
> instruction, rename it to aarch64_imm_common().

nit, but I think "label_imm_common()" would be a better name.

Anyway, I checked the encodings and the code looks good, so:

Acked-by: Will Deacon <will@...nel.org>

Will