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

From: Puranjay Mohan <puranjay12@gmail.com>
To: "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc: Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <martin.lau@linux.dev>,
	Song Liu <song@kernel.org>,
	Yonghong Song <yonghong.song@linux.dev>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>,
	Jiri Olsa <jolsa@kernel.org>,
	Shubham Bansal <illusionist.neo@gmail.com>,
	Mykola Lysenko <mykolal@fb.com>, Shuah Khan <shuah@kernel.org>,
	bpf@vger.kernel.org, linux-kselftest@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH bpf-next 6/8] arm32, bpf: add support for 64 bit division instruction
Date: Wed, 06 Sep 2023 19:19:50 +0000	[thread overview]
Message-ID: <mb61p8r9j2jop.fsf@amazon.com> (raw)
In-Reply-To: <ZPjLSHG7JToLWvmT@shell.armlinux.org.uk> (Russell King's message of "Wed, 6 Sep 2023 19:56:08 +0100")

On Wed, Sep 06 2023, Russell King (Oracle) wrote:

> On Wed, Sep 06, 2023 at 09:29:19AM +0000, Puranjay Mohan wrote:
>> On Tue, Sep 05 2023, Russell King (Oracle) wrote:
>> 
>> > On Tue, Sep 05, 2023 at 09:06:19PM +0000, Puranjay Mohan wrote:
>> 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);
>>  	}
>
> Are you sure all four states are possible?

ohh!

I just realized that the last else will never run.
rd[1] can never be equal to both ARM_R0 and ARM_R2.
Will fix it in V3 as I already sent out the V2.

I need to learn to leave patches on the list for few days before re-spinning.

Thanks,
Puranjay