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Date:   Tue, 9 Feb 2021 17:14:20 +0100
From:   Thomas Bogendoerfer <tsbogend@...ha.franken.de>
To:     "Maciej W. Rozycki" <macro@...am.me.uk>
Cc:     Jinyang He <hejinyang@...ngson.cn>,
        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 Sun, Feb 07, 2021 at 10:31:38PM +0100, 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.
> 
>  Thomas, please revert this change as I can see you've already taken it.  
> It's plain wrong.

It's now reverted in mips-next.

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.                                                [ RFC1925, 2.3 ]

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