linux-kernel - Re: [PATCH v2 2/4] MIPS: microMIPS: Fix the judgment of mm_jr16_op and mm_jalr

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Date:   Mon, 8 Feb 2021 09:13:13 +0800
From:   Jinyang He <hejinyang@...ngson.cn>
To:     "Maciej W. Rozycki" <macro@...am.me.uk>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>
Cc:     Jiaxun Yang <jiaxun.yang@...goat.com>, linux-mips@...r.kernel.org,
        linux-kernel@...r.kernel.org, Paul Burton <paulburton@...nel.org>,
        Jun-Ru Chang <jrjang@...ltek.com>
Subject: Re: [PATCH v2 2/4] MIPS: microMIPS: Fix the judgment of mm_jr16_op
 and mm_jalr_op

On 02/08/2021 05:31 AM, Maciej W. Rozycki wrote:

> On Thu, 21 Jan 2021, Jinyang He wrote:
>
>> mm16_r5_format.rt is 5 bits, so directly judge the value if equal or not.
>> mm_jalr_op requires 7th to 16th bits. These 10 which bits generated by
>   The minor opcode extension field is comprised of bits 15:6, not 16:7 as
> your description suggests.  Please be accurate with statements.
>
>> shifting u_format.uimmediate by 6 may be affected by sign extension.
>   Why?  The `uimmediate' bit-field member is unsigned for a reason.  No
> sign-extension is made on unsigned data with the right-shift operation.
>
>> Thus, take out the 10 bits for comparison.
>>
>> Without this patch, errors may occur, such as these bits are all ones.
>   How did you come to this conclusion?
>
>> diff --git a/arch/mips/kernel/process.c b/arch/mips/kernel/process.c
>> index d737234..74d7fd8 100644
>> --- a/arch/mips/kernel/process.c
>> +++ b/arch/mips/kernel/process.c
>> @@ -292,8 +292,8 @@ static inline int is_jump_ins(union mips_instruction *ip)
>>   	 * microMIPS is kind of more fun...
>>   	 */
>>   	if (mm_insn_16bit(ip->word >> 16)) {
>> -		if ((ip->mm16_r5_format.opcode == mm_pool16c_op &&
>> -		    (ip->mm16_r5_format.rt & mm_jr16_op) == mm_jr16_op))
>> +		if (ip->mm16_r5_format.opcode == mm_pool16c_op &&
>> +		    ip->mm16_r5_format.rt == mm_jr16_op)
>>   			return 1;
>>   		return 0;
>>   	}
>   Code style changes should be submitted on their own as separate patches.
>
>> @@ -305,7 +305,7 @@ static inline int is_jump_ins(union mips_instruction *ip)
>>   	if (ip->r_format.opcode != mm_pool32a_op ||
>>   			ip->r_format.func != mm_pool32axf_op)
>>   		return 0;
>> -	return ((ip->u_format.uimmediate >> 6) & mm_jalr_op) == mm_jalr_op;
>> +	return ((ip->u_format.uimmediate >> 6) & GENMASK(9, 0)) == mm_jalr_op;
>   You've now excluded JALR.HB, JALRS, and JALRS.HB instructions.  The mask
> was there for a reason.  If you can't be bothered to verify microMIPS
> changes say with QEMU, then at the very least please check documentation.
> The intent of this code is clear and these instructions are even spelled
> out explicitly in the comment at the top.
It's my fault. :-(

How amazing the opcode design is!

Thanks,
Jinyang

>   Thomas, please revert this change as I can see you've already taken it.
> It's plain wrong.
>
>    Maciej