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SubjectRe: [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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