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

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Date:   Mon, 10 Apr 2023 22:16:31 +0800
From:   WANG Xuerui <kernel@...0n.name>
To:     Tiezhu Yang <yangtiezhu@...ngson.cn>,
        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 2023/4/7 20:02, Tiezhu Yang wrote:
> 
> 
> 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)

Sorry, I meant *not* ditching the union-typed parameters. IMO they 
should behave the same codegen-wise (i.e. unchanged performance), and 
have the benefit of being clearly typed unlike plain u32's.

-- 
WANG "xen0n" Xuerui

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