[PATCH v5 0/9] Add OPTPROBES feature on RISCV

Chen Guokai
2022-12-24T15:22:37Z

Add jump optimization support for RISC-V.

Replaces ebreak instructions used by normal kprobes with an
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
Deaconappends the lower 20 bits to form a 32 bit immediate. Note that
returns 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 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
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. 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 of searching free register, evaluate
the ratio of optimizaion, 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 entire feature
into some individual patches.

v5:
1. Correct known nits
2. Enable the usage of unused caller-saved registers
3. Append an efficiency test result on Gem 5

v4:
Correct the sequence of Signed-off-by and Co-developed-by.

v3:
1. Support of hybrid RVI and RVC kernel binary.
2. Refactor out entire feature into some individual patches.

v2:
1. Adjust comments
2. Remove improper copyright
3. Clean up format issues that is no common practice
4. Extract common definition of instruction decoder
5. Fix race issue in SMP platform.

v1:
Chen Guokai contribute the basic functionality code.

Chen Guokai (1):
riscv/kprobe: Search free registers from unused caller-saved ones

Liao Chang (8):
riscv/kprobe: Prepare the skeleton to implement RISCV OPTPROBES
feature
riscv/kprobe: Allocate detour buffer from module area
riscv/kprobe: Prepare the skeleton to prepare optimized kprobe
riscv/kprobe: Add common RVI and RVC instruction decoder code
riscv/kprobe: Search free register(s) to clobber for 'AUIPC/JALR'
riscv/kprobe: Add code to check if kprobe can be optimized
riscv/kprobe: Prepare detour buffer for optimized kprobe
riscv/kprobe: Patch AUIPC/JALR pair to optimize kprobe

arch/riscv/Kconfig | 1 +
arch/riscv/include/asm/bug.h | 5 +-
arch/riscv/include/asm/kprobes.h | 49 ++
arch/riscv/include/asm/patch.h | 1 +
arch/riscv/kernel/patch.c | 23 +-
arch/riscv/kernel/probes/Makefile | 1 +
arch/riscv/kernel/probes/decode-insn.h | 153 +++++
arch/riscv/kernel/probes/kprobes.c | 24 +
arch/riscv/kernel/probes/opt.c | 693 ++++++++++++++++++++++
arch/riscv/kernel/probes/opt_trampoline.S | 137 +++++
arch/riscv/kernel/probes/simulate-insn.h | 41 ++
11 files changed, 1123 insertions(+), 5 deletions(-)
create mode 100644 arch/riscv/kernel/probes/opt.c
create mode 100644 arch/riscv/kernel/probes/opt_trampoline.S

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2.34.1