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: Palmer Dabbelt <palmer@rivosinc.com>