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* [PATCH 0/8] riscv: improve boot time isa extensions handling
@ 2022-10-06  7:08 Jisheng Zhang
  2022-10-06  7:08 ` [PATCH 1/8] riscv: move riscv_noncoherent_supported() out of ZICBOM probe Jisheng Zhang
                   ` (9 more replies)
  0 siblings, 10 replies; 35+ messages in thread
From: Jisheng Zhang @ 2022-10-06  7:08 UTC (permalink / raw)
  To: Paul Walmsley, Palmer Dabbelt, Albert Ou; +Cc: linux-riscv, linux-kernel

Generally, riscv ISA extensions are fixed for any specific hardware
platform, that's to say, the hart features won't change any more
after booting, this chacteristic make it straightforward to use
static branch to check one 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/


Jisheng Zhang (8):
  riscv: move riscv_noncoherent_supported() out of ZICBOM probe
  riscv: cpufeature: detect RISCV_ALTERNATIVES_EARLY_BOOT earlier
  riscv: hwcap: make ISA extension ids can be used in asm
  riscv: cpufeature: extend riscv_cpufeature_patch_func to all ISA
    extensions
  riscv: introduce riscv_has_extension_[un]likely()
  riscv: fpu: switch has_fpu() to riscv_has_extension_likely()
  riscv: cpu_relax: switch to riscv_has_extension_likely()
  riscv: remove riscv_isa_ext_keys[] array and related usage

 arch/riscv/include/asm/errata_list.h    |  9 +--
 arch/riscv/include/asm/hwcap.h          | 94 ++++++++++++++-----------
 arch/riscv/include/asm/switch_to.h      |  3 +-
 arch/riscv/include/asm/vdso/processor.h |  2 +-
 arch/riscv/kernel/cpufeature.c          | 78 +++-----------------
 arch/riscv/kernel/setup.c               |  4 ++
 6 files changed, 71 insertions(+), 119 deletions(-)

-- 
2.37.2


^ permalink raw reply	[flat|nested] 35+ messages in thread

end of thread, other threads:[~2022-10-30 16:16 UTC | newest]

Thread overview: 35+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2022-10-06  7:08 [PATCH 0/8] riscv: improve boot time isa extensions handling Jisheng Zhang
2022-10-06  7:08 ` [PATCH 1/8] riscv: move riscv_noncoherent_supported() out of ZICBOM probe Jisheng Zhang
2022-10-06 13:10   ` Andrew Jones
2022-10-06 17:44     ` Andrew Jones
2022-10-07  9:18   ` Heiko Stübner
2022-10-08 13:06   ` Conor Dooley
2022-10-08 13:59     ` Jisheng Zhang
2022-10-13  5:37       ` Conor Dooley
2022-10-06  7:08 ` [PATCH 2/8] riscv: cpufeature: detect RISCV_ALTERNATIVES_EARLY_BOOT earlier Jisheng Zhang
2022-10-06 13:12   ` Andrew Jones
2022-10-07  9:38   ` Heiko Stübner
2022-10-06  7:08 ` [PATCH 3/8] riscv: hwcap: make ISA extension ids can be used in asm Jisheng Zhang
2022-10-06 13:15   ` Andrew Jones
2022-10-07  9:22   ` Heiko Stübner
2022-10-06  7:08 ` [PATCH 4/8] riscv: cpufeature: extend riscv_cpufeature_patch_func to all ISA extensions Jisheng Zhang
2022-10-06 13:31   ` Andrew Jones
2022-10-07 11:54   ` Heiko Stübner
2022-10-13 13:28     ` Heiko Stuebner
2022-10-06  7:08 ` [PATCH 5/8] riscv: introduce riscv_has_extension_[un]likely() Jisheng Zhang
2022-10-06 13:36   ` Andrew Jones
2022-10-06  7:08 ` [PATCH 6/8] riscv: fpu: switch has_fpu() to riscv_has_extension_likely() Jisheng Zhang
2022-10-06 13:36   ` Andrew Jones
2022-10-07 15:11   ` Heiko Stübner
2022-10-06  7:08 ` [PATCH 7/8] riscv: cpu_relax: switch " Jisheng Zhang
2022-10-06 13:28   ` kernel test robot
2022-10-06 13:37   ` Andrew Jones
2022-10-07 15:12   ` Heiko Stübner
2022-10-07 18:14   ` kernel test robot
2022-10-06  7:08 ` [PATCH 8/8] riscv: remove riscv_isa_ext_keys[] array and related usage Jisheng Zhang
2022-10-06 13:38   ` Andrew Jones
2022-10-07 15:12   ` Heiko Stübner
2022-10-13 16:20 ` [PATCH 0/8] riscv: improve boot time isa extensions handling Andrew Jones
2022-10-29  9:56 ` Andrew Jones
2022-10-29 11:38   ` Jisheng Zhang
2022-10-30 16:03   ` Jisheng Zhang

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