[Deepin-Kernel-SIG] [linux 6.18-y] [Fromlist] Improve RISC-V strnlen() and strchr() performance

[Avenger-285714]

Summary by Sourcery

Add optimized RISC-V implementations of strnlen() and strchr() and integrate them into the architecture-specific string library and purgatory build.

New Features:

• Provide an architecture-specific strnlen() implementation for RISC-V with optional Zbb-accelerated variant.
• Provide an architecture-specific strchr() implementation for RISC-V.

Enhancements:

• Wire the new RISC-V strnlen() and strchr() implementations into the core lib and purgatory build so they are used when KASAN is disabled.

[sourcery-ai]

Reviewer's Guide

Adds RISC-V assembly implementations of strnlen() and strchr(), wires them into the generic RISC-V string infrastructure and purgatory, and provides an optimized Zbb-based strnlen() variant when the ISA and toolchain support it.

Flow diagram for RISC-V strchr implementation

flowchart TD
  Start[Start strchr
  a0 = s, a1 = c] --> MaskChar
  MaskChar[Mask search char
  a1 = a1 & 0xff] --> Loop

  Loop[Load byte t0 = *a0] --> FoundCheck
  FoundCheck{t0 == a1?} -->|yes| ReturnPtr
  FoundCheck -->|no| NullCheck

  NullCheck{t0 == 0?} -->|yes| ReturnNull
  NullCheck -->|no| IncPtr

  IncPtr[a0 = a0 + 1] --> Loop

  ReturnPtr[Return a0] --> End
  ReturnNull[Set a0 = 0
  Return NULL] --> End

  End[End strchr]

Loading

File-Level Changes

Change	Details	Files
Enable use of RISC-V-specific strnlen() and strchr() implementations in both the main kernel and purgatory when KASAN is disabled.	Extend purgatory-y object list to include strnlen.o and strchr.o alongside existing string routines when KASAN is not enabled Add explicit build rules in the RISC-V purgatory Makefile to assemble strnlen.S and strchr.S Include strnlen.o and strchr.o in arch/riscv/lib/Makefile’s lib-y objects under non-KASAN builds	arch/riscv/purgatory/Makefile arch/riscv/lib/Makefile
Declare RISC-V architecture-specific prototypes for strnlen() and strchr() so the generic code uses these implementations instead of the generic C versions.	Define __HAVE_ARCH_STRNLEN and declare the strnlen() prototype in the RISC-V asm string header Define __HAVE_ARCH_STRCHR and declare the strchr() prototype in the RISC-V asm string header	arch/riscv/include/asm/string.h
Introduce an efficient RISC-V assembly implementation of strnlen() with an optional Zbb-accelerated path and export it for kernel-wide use, including PI aliasing.	Implement a simple byte-wise strnlen() loop that respects the max length bound and returns the number of bytes before NUL or count Provide a Zbb-optimized strnlen_zbb variant that uses word-sized operations, bit-manipulation (orc.b, clz/ctz) and alignment handling when the RISCV_ISA_ZBB extension and toolchain support are present Use __ALTERNATIVE_CFG to branch to the Zbb variant when available at runtime while keeping a generic path otherwise Add SYM_FUNC_END, __pi_strnlen alias, and EXPORT_SYMBOL(strnlen) so the symbol is visible and properly annotated	arch/riscv/lib/strnlen.S
Add a lightweight RISC-V assembly implementation of strchr() and export it with a weak PI alias for use across the kernel.	Implement a byte-wise strchr() loop that scans for the first occurrence of a character (masked to 8 bits), terminates on match or NUL, and returns a pointer or NULL Annotate the function with SYM_FUNC_START/END, provide a weak __pi_strchr alias, and EXPORT_SYMBOL(strchr)	arch/riscv/lib/strchr.S

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[sourcery-ai]

Hey - I've left some high level feedback:

• The new __pi_strnlen alias is defined with SYM_FUNC_ALIAS while __pi_strchr uses SYM_FUNC_ALIAS_WEAK; it would be good to align this with how other RISC-V string helpers are defined (e.g., strlen, strcmp) so the __pi_* aliases all follow a consistent strength and override behavior.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The new `__pi_strnlen` alias is defined with `SYM_FUNC_ALIAS` while `__pi_strchr` uses `SYM_FUNC_ALIAS_WEAK`; it would be good to align this with how other RISC-V string helpers are defined (e.g., `strlen`, `strcmp`) so the `__pi_*` aliases all follow a consistent strength and override behavior.

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[Copilot]

Pull request overview

This PR adds RISC-V architecture-specific implementations of strnlen() (including a Zbb-accelerated variant) and strchr(), and wires them into both the normal RISC-V lib build and the kexec purgatory build so the generic C implementations can be bypassed when KASAN is not enabled.

Changes:

• Add arch/riscv/lib/strnlen.S with an optional Zbb fast-path selected via alternatives.
• Add arch/riscv/lib/strchr.S and expose both functions as arch-provided string routines.
• Update RISC-V lib and purgatory Makefiles plus asm/string.h to build/use these routines when KASAN isn’t in use.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
arch/riscv/purgatory/Makefile	Link strnlen.o/strchr.o into purgatory and add build rules for the new assembly sources.
arch/riscv/lib/strnlen.S	New RISC-V strnlen() implementation with Zbb alternative path.
arch/riscv/lib/strchr.S	New RISC-V strchr() implementation and symbol alias/export plumbing.
arch/riscv/lib/Makefile	Build strnlen.o/strchr.o as part of arch/riscv/lib/ when KASAN is disabled.
arch/riscv/include/asm/string.h	Advertise __HAVE_ARCH_STRNLEN / __HAVE_ARCH_STRCHR and declare the new symbols.

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[Copilot]

Comment typo: "Base on" should be "Based on".

Suggested change

	* Base on arch/riscv/lib/strlen.S
	* Based on arch/riscv/lib/strlen.S

[Copilot]

__pi_strchr is declared as a weak alias to strchr, which is inconsistent with the other RISC-V string routines in this directory (they use SYM_FUNC_ALIAS(__pi_*, *)). Unless there is a specific need for weak binding here, prefer a non-weak alias for consistency and to avoid surprising symbol resolution behavior.

Suggested change

	SYM_FUNC_ALIAS_WEAK(__pi_strchr, strchr)
	SYM_FUNC_ALIAS(__pi_strchr, strchr)

[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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