On 2023/2/2 16:42, Huacai Chen wrote:
Introduce Kconfig option ARCH_STRICT_ALIGN to make -mstrict-align be
configurable.
Not all LoongArch cores support h/w unaligned access, we can use the
-mstrict-align build parameter to prevent unaligned accesses.
This option is disabled by default to optimise for performance, but you
can enabled it manually if you want to run kernel on systems without h/w
unaligned access support.
It's customary to accompany "performance-related" changes like this with
some benchmark numbers and concrete use cases where this would be
profitable. Especially given that arch/loongarch developer and user base
is relatively small, we probably don't want to allow customization of
such a low-level characteristic. In general kernel performance does not
vary much with compiler flags like this, so I'd really hope to see some
numbers here to convince people that this is *really* providing gains.
Also, defaulting to emitting unaligned accesses would mean those future,
likely embedded models (and AFAIK some existing models that haven't
reached GA yet) would lose support with the defconfig. Which means
downstream packagers that care about those use cases would have one more
non-default, non-generic option to carry within their Kconfig. We
probably don't want to repeat the history of other architectures (think
arch/arm or arch/mips) where there wasn't really generic builds and
board-specific tweaks proliferated.
Signed-off-by: Huacai Chen <chenhuacai@xxxxxxxxxxx>
---
arch/loongarch/Kconfig | 10 ++++++++++
arch/loongarch/Makefile | 2 ++
2 files changed, 12 insertions(+)
diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
index 9cc8b84f7eb0..7470dcfb32f0 100644
--- a/arch/loongarch/Kconfig
+++ b/arch/loongarch/Kconfig
@@ -441,6 +441,16 @@ config ARCH_IOREMAP
protection support. However, you can enable LoongArch DMW-based
ioremap() for better performance.
+config ARCH_STRICT_ALIGN
+ bool "Enable -mstrict-align to prevent unaligned accesses"
+ help
+ Not all LoongArch cores support h/w unaligned access, we can use
+ -mstrict-align build parameter to prevent unaligned accesses.
+
+ This is disabled by default to optimise for performance, you can
+ enabled it manually if you want to run kernel on systems without
+ h/w unaligned access support.
+
config KEXEC
bool "Kexec system call"
select KEXEC_CORE
diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
index 4402387d2755..ccfb52700237 100644
--- a/arch/loongarch/Makefile
+++ b/arch/loongarch/Makefile
@@ -91,10 +91,12 @@ KBUILD_CPPFLAGS += -DVMLINUX_LOAD_ADDRESS=$(load-y)
# instead of .eh_frame so we don't discard them.
KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
+ifdef CONFIG_ARCH_STRICT_ALIGN
# Don't emit unaligned accesses.
# Not all LoongArch cores support unaligned access, and as kernel we can't
# rely on others to provide emulation for these accesses.
KBUILD_CFLAGS += $(call cc-option,-mstrict-align)
+endif >
KBUILD_CFLAGS += -isystem $(shell $(CC) -print-file-name=include)
--
WANG "xen0n" Xuerui
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