1. 02 Feb, 2023 1 commit
    • Palmer Dabbelt's avatar
      Merge patch series "riscv: improve boot time isa extensions handling" · 9daca9a5
      Palmer Dabbelt authored
      Jisheng Zhang <jszhang@kernel.org> says:
      
      Generally, riscv ISA extensions are fixed for any specific hardware
      platform, so a hart's features won't change after booting, this
      chacteristic makes it straightforward to use a static branch to check
      a specific ISA extension is supported or not to optimize performance.
      
      However, some ISA extensions such as SVPBMT and ZICBOM are handled
      via. the alternative sequences.
      
      Basically, for ease of maintenance, we prefer to use static branches
      in C code, but recently, Samuel found that the static branch usage in
      cpu_relax() breaks building with CONFIG_CC_OPTIMIZE_FOR_SIZE[1]. As
      Samuel pointed out, "Having a static branch in cpu_relax() is
      problematic because that function is widely inlined, including in some
      quite complex functions like in the VDSO. A quick measurement shows
      this static branch is responsible by itself for around 40% of the jump
      table."
      
      Samuel's findings pointed out one of a few downsides of static branches
      usage in C code to handle ISA extensions detected at boot time:
      static branch's metadata in the __jump_table section, which is not
      discarded after ISA extensions are finalized, wastes some space.
      
      I want to try to solve the issue for all possible dynamic handling of
      ISA extensions at boot time. Inspired by Mark[2], this patch introduces
      riscv_has_extension_*() helpers, which work like static branches but
      are patched using alternatives, thus the metadata can be freed after
      patching.
      
      [1]https://lore.kernel.org/linux-riscv/20220922060958.44203-1-samuel@sholland.org/
      [2]https://lore.kernel.org/linux-arm-kernel/20220912162210.3626215-8-mark.rutland@arm.com/
      [3]https://lore.kernel.org/linux-riscv/20221130225614.1594256-1-heiko@sntech.de/
      
      * b4-shazam-merge:
        riscv: remove riscv_isa_ext_keys[] array and related usage
        riscv: KVM: Switch has_svinval() to riscv_has_extension_unlikely()
        riscv: cpu_relax: switch to riscv_has_extension_likely()
        riscv: alternative: patch alternatives in the vDSO
        riscv: switch to relative alternative entries
        riscv: module: Add ADD16 and SUB16 rela types
        riscv: module: move find_section to module.h
        riscv: fpu: switch has_fpu() to riscv_has_extension_likely()
        riscv: introduce riscv_has_extension_[un]likely()
        riscv: cpufeature: extend riscv_cpufeature_patch_func to all ISA extensions
        riscv: hwcap: make ISA extension ids can be used in asm
        riscv: cpufeature: detect RISCV_ALTERNATIVES_EARLY_BOOT earlier
        riscv: move riscv_noncoherent_supported() out of ZICBOM probe
      
      Link: https://lore.kernel.org/r/20230128172856.3814-1-jszhang@kernel.orgSigned-off-by: default avatarPalmer Dabbelt <palmer@rivosinc.com>
      9daca9a5
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