linux-kernel - Re: [RFC PATCH 2/3] LoongArch: Add larch_insn_gen

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Message-ID: <9433a12a-b24b-4438-fdc8-0213522c71ba@loongson.cn>
Date:   Fri, 7 Apr 2023 20:02:30 +0800
From:   Tiezhu Yang <yangtiezhu@...ngson.cn>
To:     WANG Xuerui <kernel@...0n.name>,
        Youling Tang <tangyouling@...ngson.cn>
Cc:     Huacai Chen <chenhuacai@...nel.org>, loongarch@...ts.linux.dev,
        linux-kernel@...r.kernel.org, loongson-kernel@...ts.loongnix.cn
Subject: Re: [RFC PATCH 2/3] LoongArch: Add larch_insn_gen_break() to generate
 break insn



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