From: "Björn Töpel" <bjorn@kernel.org>
To: Chen Guokai <chenguokai17@mails.ucas.ac.cn>,
paul.walmsley@sifive.com, palmer@dabbelt.com,
aou@eecs.berkeley.edu, rostedt@goodmis.org, mingo@redhat.com,
sfr@canb.auug.org.au
Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
liaochang1@huawei.com,
Chen Guokai <chenguokai17@mails.ucas.ac.cn>
Subject: Re: [PATCH v6 00/13] Add OPTPROBES feature on RISCV
Date: Wed, 01 Feb 2023 14:29:17 +0100 [thread overview]
Message-ID: <87zg9xtsya.fsf@all.your.base.are.belong.to.us> (raw)
In-Reply-To: <20230127130541.1250865-1-chenguokai17@mails.ucas.ac.cn>
Chen Guokai <chenguokai17@mails.ucas.ac.cn> writes:
> Add jump optimization support for RISC-V.
>
> Replaces ebreak instructions used by normal kprobes with an AUIPC/JALR
> instruction pair with 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
> Deaconappends the lower 20 bits to form a 32 bits immediate. Note that
> returns from probe handler require another free register. As kprobes
> can appear almost anywhere inside the kernel, the free register should
> be found generically, not depending on calling convention or any other
> regulations.
>
> The algorithm for finding the free register is inspired by the register
> renaming in modern processors. From the perspective of register
> renaming, a register could be represented as two different registers if
> two neighbor instructions both write to it but no one ever reads it.
> 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% of instructions of the kernel (default
> config) is capable of being replaced i.e. one free register can be found
> at both the start and end of replaced instruction pairs while the
> replaced instructions can be directly executed. We also made an
> efficiency test on Gem 5 RISCV which shows a more than 5x speedup on
> breakpoint-based implementation.
>
> Contribution:
> Chen Guokai invents the algorithm for searching free register, evaluate
> the ratio of optimization, the basic function support RVI kernel binary.
> Liao Chang adds the support for hybrid RVI and RVC kernel binary, fix
> some bugs with different kernel configure, refactor out the entire
> feature into some individual patches.
Thank you for continuing to work on this series! I took it for a spin,
and it worked nicely on my QEMU setup.
It would be nice to have it run on some *actual* hardware as well. :-)
I have some additional comments on the series, but I'll add those to the
relevant patch. It's mostly minor things!
Björn
prev parent reply other threads:[~2023-02-01 13:30 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-27 13:05 Chen Guokai
2023-01-27 13:05 ` [PATCH v6 01/13] riscv/kprobe: Prepare the skeleton to implement RISCV OPTPROBES Chen Guokai
2023-01-27 13:05 ` [PATCH v6 02/13] riscv/kprobe: Allocate detour buffer from module region Chen Guokai
2023-01-27 13:05 ` [PATCH v6 03/13] riscv/kprobe: Add skeleton for preparing optimized kprobe Chen Guokai
2023-01-27 13:05 ` [PATCH v6 04/13] riscv/kprobe: Add common RVI and RVC instruction decoder code Chen Guokai
2023-02-01 13:29 ` Björn Töpel
2023-02-02 10:16 ` Conor Dooley
2023-01-27 13:05 ` [PATCH v6 05/13] riscv/kprobe: Introduce free register(s) searching algorithm Chen Guokai
2023-01-27 13:05 ` [PATCH v6 06/13] riscv/kprobe: Add code to check if kprobe can be optimized Chen Guokai
2023-02-01 13:30 ` Björn Töpel
2023-01-27 13:05 ` [PATCH v6 07/13] riscv/kprobe: Prepare detour buffer for optimized kprobe Chen Guokai
2023-02-01 13:30 ` Björn Töpel
2023-01-27 13:05 ` [PATCH v6 08/13] riscv/kprobe: Patch AUIPC/JALR pair to optimize kprobe Chen Guokai
2023-02-01 13:31 ` Björn Töpel
2023-01-27 13:05 ` [PATCH v6 09/13] riscv/kprobe: Search free registers from unused caller-saved ones Chen Guokai
2023-02-01 13:31 ` Björn Töpel
2023-02-02 9:08 ` Conor Dooley
2023-01-27 13:05 ` [PATCH v6 10/13] riscv/kprobe: Add instruction boundary check for RVI/RVC hybrid kernel Chen Guokai
2023-01-27 13:05 ` [PATCH v6 11/13] riscv/kprobe: Fix instruction simulation of JALR Chen Guokai
2023-01-31 12:51 ` Björn Töpel
2023-01-27 13:05 ` [PATCH v6 12/13] riscv/kprobe: Move exception related symbols to .kprobe_blacklist Chen Guokai
2023-02-01 13:30 ` Björn Töpel
2023-01-27 13:05 ` [PATCH v6 13/13] selftest/kprobes: Add testcase for kprobe SYM[+offs] Chen Guokai
2023-01-30 12:31 ` [PATCH v6 00/13] Add OPTPROBES feature on RISCV Björn Töpel
2023-01-30 14:38 ` Xim
2023-04-26 18:01 ` Palmer Dabbelt
2023-02-01 13:29 ` Björn Töpel [this message]
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