Re: [PATCH v2 2/4] MIPS: microMIPS: Fix the judgment of mm_jr16_op and mm_jalr_op

[Maciej W. Rozycki]

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.

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

  Maciej