Date:   Wed, 31 Aug 2022 16:28:56 +0800
From:   "liaochang (A)" <liaochang1@...wei.com>
To:     <Conor.Dooley@...rochip.com>, <chenguokai17@...ls.ucas.ac.cn>,
        <paul.walmsley@...ive.com>, <palmer@...belt.com>,
        <aou@...s.berkeley.edu>, <rostedt@...dmis.org>, <mingo@...hat.com>,
        <sfr@...b.auug.org.au>
CC:     <linux-riscv@...ts.infradead.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arch/riscv: kprobes: implement optprobes



在 2022/8/31 15:52, Conor.Dooley@...rochip.com 写道:
> On 31/08/2022 08:49, liaochang (A) wrote:
>> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
>>
>> 在 2022/8/31 15:24, Conor.Dooley@...rochip.com 写道:
>>> Hey Chen,
>>>
>>> FYI there is a build warning with this patch:
>>> arch/riscv/kernel/probes/opt.c:34:27: warning: no previous prototype for 'can_kprobe_direct_exec' [-Wmissing-prototypes]
>>>      34 | enum probe_insn __kprobes can_kprobe_direct_exec(kprobe_opcode_t *addr)
>>>
>>> Also, if you run scripts/checkpatch.pl --strict, it will have a
>>> few complaints about code style for you too. Other than that, I
>>> have a few comments for you below:
>>>
>>> On 31/08/2022 05:10, Chen Guokai wrote:
>>>> [You don't often get email from chenguokai17@...ls.ucas.ac.cn. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ]
>>>>
>>>> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
>>>>
>>>> This patch adds jump optimization support for RISC-V.
>>>
>>> s/This patch adds/Add
>>>
>>>>
>>>> This patch replaces ebreak instructions used by normal kprobes with an
>>>
>>> s/This patch replaces/Replace
>>>
>>>> auipc+jalr instruction pair, at the aim of suppressing the probe-hit
>>>> overhead.
>>>>
>>>> All known optprobe-capable RISC architectures have been using a single
>>>> jump or branch instructions while this patch chooses not. RISC-V has a
>>>> quite limited jump range (4KB or 2MB) for both its branch and jump
>>>> instructions, which prevent optimizations from supporting probes that
>>>> spread all over the kernel.
>>>>
>>>> Auipc-jalr instruction pair is introduced with a much wider jump range
>>>> (4GB), where auipc loads the upper 12 bits to a free register and jalr
>>>> appends the lower 20 bits to form a 32 bit immediate. Note that returning
>>>> from probe handler requires another free register. As kprobes can appear
>>>> almost anywhere inside the kernel, the free register should be found in a
>>>> generic way, not depending on calling convension or any other regulations.
>>>
>>> convention
>>>
>>>>
>>>> The algorithm for finding the free register is inspired by the regiter
>>>
>>> register
>>>
>>>> renaming in modern processors. From the perspective of register renaming, a
>>>> register could be represented as two different registers if two neighbour
>>>> instructions both write to it but no one ever reads. Extending this fact,
>>>> a register is considered to be free if there is no read before its next
>>>> write in the execution flow. We are free to change its value without
>>>> interfering normal execution.
>>>>
>>>> Static analysis shows that 51% instructions of the kernel (default config)
>>>> is capable of being replaced i.e. two free registers can be found at both
>>>> the start and end of replaced instruction pairs while the replaced
>>>> instructions can be directly executed.
>>>>
>>>> Signed-off-by: Chen Guokai <chenguokai17@...ls.ucas.ac.cn>
>>>> Signed-off-by: Liao Chang <liaochang1@...wei.com>
>>>
>>> What does Liao have to do with this patch?
>> I just provide some suggestion to Chen Guokai during development ;)
>> please remove my info from Signed-off-by tag.
> 
> Does that mean that the "copyright 2022 Huawei" is also not accurate?
Inaccurate, please remove "copyright 2022 Huawei",thanks for checking.

>   
> 
> 

-- 
BR,
Liao, Chang

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