[PATCH] arch/riscv: kprobes: implement optprobes

[Conor.Dooley at microchip.com]

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 at microchip.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 at mails.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 at mails.ucas.ac.cn>
>>> Signed-off-by: Liao Chang <liaochang1 at huawei.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?