linux-kernel - [PATCH v9 0/2] riscv: Add runtime constant support

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Message-Id: <20250318-runtime_const_riscv-v9-0-ddd3534d3e8e@rivosinc.com>
Date: Tue, 18 Mar 2025 17:38:44 -0700
From: Charlie Jenkins <charlie@...osinc.com>
To: Paul Walmsley <paul.walmsley@...ive.com>, 
 Palmer Dabbelt <palmer@...belt.com>, Ard Biesheuvel <ardb@...nel.org>, 
 Ben Dooks <ben.dooks@...ethink.co.uk>, 
 Pasha Bouzarjomehri <pasha@...osinc.com>, 
 Emil Renner Berthing <emil.renner.berthing@...onical.com>, 
 Alexandre Ghiti <alexghiti@...osinc.com>, 
 Steven Rostedt <rostedt@...dmis.org>, 
 Masami Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>, 
 Albert Ou <aou@...s.berkeley.edu>, Peter Zijlstra <peterz@...radead.org>, 
 Josh Poimboeuf <jpoimboe@...nel.org>, Jason Baron <jbaron@...mai.com>
Cc: linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
 linux-trace-kernel@...r.kernel.org, Charlie Jenkins <charlie@...osinc.com>
Subject: [PATCH v9 0/2] riscv: Add runtime constant support

Ard brought this to my attention in this patch [1].

I benchmarked this patch on the Nezha D1 (which does not contain Zba or
Zbkb so it uses the default algorithm) by navigating through a large
directory structure. I created a 1000-deep directory structure and then
cd and ls through it. With this patch there was a 0.57% performance
improvement.

[1] https://lore.kernel.org/lkml/CAMj1kXE4DJnwFejNWQu784GvyJO=aGNrzuLjSxiowX_e7nW8QA@mail.gmail.com/

Signed-off-by: Charlie Jenkins <charlie@...osinc.com>
---
Changes in v9:
- Fix bug where stale register data may be used when an lui is replaced
  with a nop. In the following addiw, add to register x0 instead of the
  stale register to resolve.
- Add locks for text_mutex before using patch_insn_write()
- Link to v8: https://lore.kernel.org/r/20250305-runtime_const_riscv-v8-1-fa66f3468dac@rivosinc.com

Changes in v8:
- Rebase to linux v6.14-rc5
- Link to v7: https://lore.kernel.org/r/20250218-runtime_const_riscv-v7-1-e431763157ff@rivosinc.com

Changes in v7:
- Added benchmarking info
- Added CONFIG_RISCV_ISA_ZBA and CONFIG_RISCV_ISA_ZBKB to check that the
  compiler supports the extensions.
- Link to v6: https://lore.kernel.org/r/20250212-runtime_const_riscv-v6-1-3ef0146b310b@rivosinc.com

Changes in v6:
- .option arch only became officially supported by clang in version 17.
  Add a config to check that and guard the alternatives uses .option
  arch.
- Link to v5: https://lore.kernel.org/r/20250203-runtime_const_riscv-v5-1-bc61736a3229@rivosinc.com

Changes in v5:
- Split instructions into 16-bit parcels to avoid alignment (Emil)
- Link to v4: https://lore.kernel.org/r/20250130-runtime_const_riscv-v4-1-2d36c41b7b9c@rivosinc.com

Changes in v4:
- Add newlines after riscv32 assembler directives
- Align instructions along 32-bit boundary (Emil)
- Link to v3: https://lore.kernel.org/r/20250128-runtime_const_riscv-v3-1-11922989e2d3@rivosinc.com

Changes in v3:
- Leverage "pack" instruction for runtime_const_ptr() to reduce hot path
  by 3 instructions if Zbkb is supported. Suggested by Pasha Bouzarjomehri (pasha@...osinc.com)
- Link to v2: https://lore.kernel.org/r/20250127-runtime_const_riscv-v2-1-95ae7cf97a39@rivosinc.com

Changes in v2:
- Treat instructions as __le32 and do proper conversions (Ben)
- Link to v1: https://lore.kernel.org/r/20250127-runtime_const_riscv-v1-1-795b023ea20b@rivosinc.com

---
Charlie Jenkins (2):
      riscv: Move nop definition to insn-def.h
      riscv: Add runtime constant support

 arch/riscv/Kconfig                     |  22 +++
 arch/riscv/include/asm/asm.h           |   1 +
 arch/riscv/include/asm/ftrace.h        |   1 -
 arch/riscv/include/asm/insn-def.h      |   2 +
 arch/riscv/include/asm/runtime-const.h | 265 +++++++++++++++++++++++++++++++++
 arch/riscv/kernel/ftrace.c             |   6 +-
 arch/riscv/kernel/jump_label.c         |   4 +-
 arch/riscv/kernel/vmlinux.lds.S        |   3 +
 8 files changed, 298 insertions(+), 6 deletions(-)
---
base-commit: 2014c95afecee3e76ca4a56956a936e23283f05b
change-id: 20250123-runtime_const_riscv-6cd854ee2817
-- 
- Charlie