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riscv: introduce RISCV_EFFICIENT_UNALIGNED_ACCESS

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Some riscv implementations such as T-HEAD's C906, C908, C910 and C920
support efficient unaligned access, for performance reason we want
to enable HAVE_EFFICIENT_UNALIGNED_ACCESS on these platforms. To
avoid performance regressions on other non efficient unaligned access
platforms, HAVE_EFFICIENT_UNALIGNED_ACCESS can't be globally selected.

To solve this problem, runtime code patching based on the detected
speed is a good solution. But that's not easy, it involves lots of
work to modify vairous subsystems such as net, mm, lib and so on.
This can be done step by step.

So let's take an easier solution: add support to efficient unaligned
access and hide the support under NONPORTABLE.

Now let's introduce RISCV_EFFICIENT_UNALIGNED_ACCESS which depends on
NONPORTABLE, if users know during config time that the kernel will be
only run on those efficient unaligned access hw platforms, they can
enable it. Obviously, generic unified kernel Image shouldn't enable it.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20231225044207.3821-2-jszhang@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
This commit is contained in:
Jisheng Zhang 2023-12-25 12:42:06 +08:00 committed by Palmer Dabbelt
parent b85ea95d08
commit b6da6cbe13
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13
arch/riscv/Kconfig
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@ -651,6 +651,19 @@ config RISCV_MISALIGNED
load/store for both kernel and userspace. When disable, misaligned
accesses will generate SIGBUS in userspace and panic in kernel.
config RISCV_EFFICIENT_UNALIGNED_ACCESS
bool "Assume the CPU supports fast unaligned memory accesses"
depends on NONPORTABLE
select HAVE_EFFICIENT_UNALIGNED_ACCESS
help
Say Y here if you want the kernel to assume that the CPU supports
efficient unaligned memory accesses. When enabled, this option
improves the performance of the kernel on such CPUs. However, the
kernel will run much more slowly, or will not be able to run at all,
on CPUs that do not support efficient unaligned memory accesses.
If unsure what to do here, say N.
endmenu # "Platform type"
menu "Kernel features"

2
arch/riscv/Makefile
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@ -108,7 +108,9 @@ KBUILD_AFLAGS_MODULE += $(call as-option,-Wa$(comma)-mno-relax)
# unaligned accesses. While unaligned accesses are explicitly allowed in the
# RISC-V ISA, they're emulated by machine mode traps on all extant
# architectures. It's faster to have GCC emit only aligned accesses.
ifneq ($(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS),y)
KBUILD_CFLAGS += $(call cc-option,-mstrict-align)
endif
ifeq ($(CONFIG_STACKPROTECTOR_PER_TASK),y)
prepare: stack_protector_prepare