linux-kernel - Re: [PATCH bpf-next 6/8] arm32, bpf: add support for 64 bit division instruction

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Date:   Wed, 06 Sep 2023 09:29:19 +0000
From:   Puranjay Mohan <puranjay12@...il.com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
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 6/8] arm32, bpf: add support for 64 bit
 division instruction

On Tue, Sep 05 2023, Russell King (Oracle) wrote:

> On Tue, Sep 05, 2023 at 09:06:19PM +0000, Puranjay Mohan wrote:
>> +cont:
>> +
>> +	/* Call appropriate function */
>> +	if (sign)
>> +		emit_mov_i(ARM_IP, op == BPF_DIV ? (u32)jit_sdiv64 : (u32)jit_smod64, ctx);
>> +	else
>> +		emit_mov_i(ARM_IP, op == BPF_DIV ? (u32)jit_udiv64 : (u32)jit_mod64, ctx);
>
> Same comment as the previous patch here.

Will fix both in next version.

>
>> +
>> +	emit_blx_r(ARM_IP, ctx);
>> +
>> +	/* Save return value */
>> +	if (rd[1] != ARM_R0) {
>> +		emit(ARM_MOV_R(rd[0], ARM_R1), ctx);
>> +		emit(ARM_MOV_R(rd[1], ARM_R0), ctx);
>> +	}
>> +
>> +	/* Recover {R1, R0} from stack if it is not Rd */
>> +	if (rd[1] != ARM_R0)
>> +		emit(ARM_POP(BIT(ARM_R0) | BIT(ARM_R1)), ctx);
>> +	else
>> +		emit(ARM_ADD_I(ARM_SP, ARM_SP, 8), ctx);
>> +
>> +	/* Recover {R3, R2} from stack if it is not Rd */
>> +	if (rd[1] != ARM_R2)
>> +		emit(ARM_POP(BIT(ARM_R2) | BIT(ARM_R3)), ctx);
>> +	else
>> +		emit(ARM_ADD_I(ARM_SP, ARM_SP, 8), ctx);
>
> 	if (rd[1] != ARM_R0) {
> 		emit(ARM_POP(BIT(ARM_R0) | BIT(ARM_R1)), ctx);
> 		emit(ARM_ADD_I(ARM_SP, ARM_SP, 8), ctx);
> 	} else if (rd[1] != ARM_R2) {
> 		emit(ARM_ADD_I(ARM_SP, ARM_SP, 8), ctx);
> 		emit(ARM_POP(BIT(ARM_R2) | BIT(ARM_R3)), ctx);
> 	} else {
> 		emit(ARM_ADD_I(ARM_SP, ARM_SP, 16), ctx);
> 	}
>
> Hmm?

Actually, there can also be a situation where rd[1] != ARM_R0 && rd[1] != ARM_R2,
so should I do it like:

 	if (rd[1] != ARM_R0 && rd[1] != ARM_R2) {
 		emit(ARM_POP(BIT(ARM_R0) | BIT(ARM_R1)), ctx);
 		emit(ARM_POP(BIT(ARM_R2) | BIT(ARM_R3)), ctx);      
 	} else if (rd[1] != ARM_R0) {
 		emit(ARM_POP(BIT(ARM_R0) | BIT(ARM_R1)), ctx);
 		emit(ARM_ADD_I(ARM_SP, ARM_SP, 8), ctx);
 	} else if (rd[1] != ARM_R2) {
 		emit(ARM_ADD_I(ARM_SP, ARM_SP, 8), ctx);
 		emit(ARM_POP(BIT(ARM_R2) | BIT(ARM_R3)), ctx);
 	} else {
 		emit(ARM_ADD_I(ARM_SP, ARM_SP, 16), ctx);
 	}

Thanks,
Puranjay