linux-kernel - Re: [PATCH v2 bpf-next 02/13] bpf: x86: Factor out emission of REX byte

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Message-ID: <20201129011405.vai66tyexpphpacb@ast-mbp>
Date:   Sat, 28 Nov 2020 17:14:05 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Brendan Jackman <jackmanb@...gle.com>
Cc:     bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Yonghong Song <yhs@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        KP Singh <kpsingh@...omium.org>,
        Florent Revest <revest@...omium.org>,
        linux-kernel@...r.kernel.org, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH v2 bpf-next 02/13] bpf: x86: Factor out emission of REX
 byte

On Fri, Nov 27, 2020 at 05:57:27PM +0000, Brendan Jackman wrote:
> The JIT case for encoding atomic ops is about to get more
> complicated. In order to make the review & resulting code easier,
> let's factor out some shared helpers.
> 
> Signed-off-by: Brendan Jackman <jackmanb@...gle.com>
> ---
>  arch/x86/net/bpf_jit_comp.c | 39 ++++++++++++++++++++++---------------
>  1 file changed, 23 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 94b17bd30e00..a839c1a54276 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -702,6 +702,21 @@ static void emit_modrm_dstoff(u8 **pprog, u32 r1, u32 r2, int off)
>  	*pprog = prog;
>  }
>  
> +/*
> + * Emit a REX byte if it will be necessary to address these registers

What is "REX byte" ?
May be rename it to maybe_emit_mod() ?

> + */
> +static void maybe_emit_rex(u8 **pprog, u32 reg_rm, u32 reg_reg, bool wide)

could you please keep original names as dst_reg/src_reg instead of reg_rm/reg_reg ?
reg_reg reads really odd and reg_rm is equally puzzling unless the reader studied
intel's manual. I didn't. All these new abbreviations are challenging for me.
> +{
> +	u8 *prog = *pprog;
> +	int cnt = 0;
> +
> +	if (wide)

what is 'wide' ? Why not to call it 'bool is_alu64' ?

> +		EMIT1(add_2mod(0x48, reg_rm, reg_reg));
> +	else if (is_ereg(reg_rm) || is_ereg(reg_reg))
> +		EMIT1(add_2mod(0x40, reg_rm, reg_reg));
> +	*pprog = prog;
> +}