Re: [RFC PATCH 2/3] LoongArch: Add larch_insn_gen_break() to generate break insn

[Tiezhu Yang]

On 04/07/2023 05:51 PM, WANG Xuerui wrote:
> On 2023/4/7 10:30, Youling Tang wrote:
> > /* snip */
> >
> > > diff --git a/arch/loongarch/kernel/kprobes.c
> > > b/arch/loongarch/kernel/kprobes.c
> > > index 08c78d2..a5c3712 100644
> > > --- a/arch/loongarch/kernel/kprobes.c
> > > +++ b/arch/loongarch/kernel/kprobes.c
> > > @@ -4,19 +4,8 @@
> > >  #include <linux/preempt.h>
> > >  #include <asm/break.h>
> > >
> > > -static const union loongarch_instruction breakpoint_insn = {
> > > -    .reg0i15_format = {
> > > -        .opcode = break_op,
> > > -        .immediate = BRK_KPROBE_BP,
> > > -    }
> > > -};
> > > -
> > > -static const union loongarch_instruction singlestep_insn = {
> > > -    .reg0i15_format = {
> > > -        .opcode = break_op,
> > > -        .immediate = BRK_KPROBE_SSTEPBP,
> > > -    }
> > > -};
> > > +#define breakpoint_insn larch_insn_gen_break(BRK_KPROBE_BP)
> > > +#define singlestep_insn larch_insn_gen_break(BRK_KPROBE_SSTEPBP)
> >
> > IMO, Defined as KPROBE_BP_INSN, KPROBE_SSTEPBP_INSN may be better.
>
> Are you suggesting to hardcode the instruction words for those two BREAK
> flavors?

I think what Youling said is:

#define KPROBE_BP_INSN         larch_insn_gen_break(BRK_KPROBE_BP)
#define KPROBE_SSTEPBP_INSN    larch_insn_gen_break(BRK_KPROBE_SSTEPBP)

> I don't think it's better because even more structured info is
> lost, and the compiler would generate the same code (if not, it's the
> compiler that's to be fixed).
>
> Actually, I don't know why this commit was necessary in the first place.
> For the very least, it consisted of two logical changes (pass around
> instruction words instead of unions; and change the BREAK insns to make
> them words) that should get split;

Yes, thanks for your suggestion, I will split it into two patches
in the next version.

> but again, the generated code should
> be identical anyway, so it seems a lot of churn for no benefit and
> reduced readability.

Define and use larch_insn_gen_break() is to avoid hardcoding the
uprobe break instruction in patch #3.

We do not like the following definitions:

#define UPROBE_SWBP_INSN	0x002a000c
#define UPROBE_XOLBP_INSN	0x002a000d

Using larch_insn_gen_break() seems better:

#define UPROBE_SWBP_INSN	larch_insn_gen_break(BRK_UPROBE_BP)
#define UPROBE_XOLBP_INSN	larch_insn_gen_break(BRK_UPROBE_XOLBP)

Thanks,
Tiezhu