[WIP] [Deepin-Kernel-SIG] [linux 6.6.y] [Upstream] riscv: Add runtime constant support

[opsiff]

mainline inclusion
from mainline-v6.15-rc1
category: performance

Implement the runtime constant infrastructure for riscv. Use this infrastructure to generate constants to be used by the d_hash() function.

This is the riscv variant of commit 94a2bc0 ("arm64: add 'runtime constant' support") and commit e3c92e8 ("runtime constants: add x86 architecture support").

[ alex: Remove trailing whitespace ]

Reviewed-by: Alexandre Ghiti alexghiti@rivosinc.com
Tested-by: Alexandre Ghiti alexghiti@rivosinc.com
Link: https://lore.kernel.org/r/20250319-runtime_const_riscv-v10-2-745b31a11d65@rivosinc.com

Conflicts:
arch/riscv/Kconfig
(cherry picked from commit a44fb57)

Summary by Sourcery

Add RISC-V runtime constant infrastructure and integrate it into the kernel image layout to support architecture-specific optimization of computed constants.

New Features:

• Introduce RISC-V-specific runtime constant support for pointers and shifts via a new asm/runtime-const.h interface.

Enhancements:

• Extend the RISC-V linker script to include the runtime constant variables section.
• Add an SRLI helper macro to abstract word-size-specific shift-right-immediate instructions used by the runtime constant logic.

[sourcery-ai]

Reviewer's Guide

Adds RISC-V runtime constant infrastructure and wires it into the linker script, providing assembly helpers and fixup routines to materialize and patch pointer and shift-immediate constants at runtime, with optional use of Zba/Zbkb ISA extensions for optimized 64-bit constant formation.

Flow diagram for RISC-V runtime constant initialization and fixup

flowchart TD
    Caller["caller of runtime_const_init(type,sym)"] --> runtime_const_init

    runtime_const_init["runtime_const_init(type,sym)"] --> runtime_const_fixup

    runtime_const_fixup["runtime_const_fixup(fn,val,start,end)"] --> Loop["[iterate runtime_* section entries]"]
    Loop --> runtime_fixup_fn

    runtime_fixup_fn["__runtime_fixup_ptr / __runtime_fixup_shift"] --> runtime_fixup_32
    runtime_fixup_fn --> runtime_fixup_caches

    runtime_fixup_32["__runtime_fixup_32(lui_parcel, addi_parcel, val)"] --> PatchedInsns["[patch lui/addi immediates]"]
    PatchedInsns --> runtime_fixup_caches

    runtime_fixup_caches["__runtime_fixup_caches(where,insns)"] --> Done["[runtime constants materialized in code]"]

Loading

File-Level Changes

Change	Details	Files
Wire runtime-constant variable section into the RISC-V vmlinux linker script so runtime-constant metadata is retained and located at link time.	Insert RUNTIME_CONST_VARIABLES section after EXIT_DATA and before per-CPU sections in the RISC-V kernel linker script	arch/riscv/kernel/vmlinux.lds.S
Extend RISC-V asm helpers with a width-dependent shift-right-immediate mnemonic for use in runtime-constant code sequences.	Add SRLI macro to select srliw on RV64 and srli on RV32 based on __REG_SEL	arch/riscv/include/asm/asm.h
Introduce RISC-V runtime-constant infrastructure to generate, record, and later patch pointer and shift constants, including optimized 64-bit constant formation paths and corresponding runtime fixup routines.	Define runtime_const_ptr() for 32-bit and 64-bit RISC-V, emitting placeholder LUI/ADDI sequences and recording their locations in dedicated runtime_ptr_* sections Provide alternative sequences for 64-bit constant formation using base instructions, Zba add.uw, or Zbkb pack, selected via ALTERNATIVE macros and ISA config options Add runtime_const_shift_right_32() to emit SRLI/SRLIW with placeholder shift amounts and record them in runtime_shift_* sections Implement runtime_const_init() and runtime_const_fixup() helper that iterate over recorded locations and patch instruction streams with final values, including cache maintenance and text_mutex synchronization Implement __runtime_fixup_32(), __runtime_fixup_ptr(), and __runtime_fixup_shift() helpers that decode/encode RISC-V instructions, update immediates, and write them back using patch_insn_write/patch_text_nosync	arch/riscv/include/asm/runtime-const.h

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[Avenger-285714]

/approve

Link: #1902

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