On Thu, Jan 25, 2018 at 12:38 PM, Yury Norov [off-list ref] wrote:

On Wed, Jan 24, 2018 at 11:28:55AM +0100, Arnd Bergmann wrote:

quoted

On Wed, Jan 24, 2018 at 10:05 AM, Yury Norov [off-list ref] wrote:

Thanks for doing this test. Looking at this I realize that this is
not the architecture feature but compiler feature. So if we add
128-bit interface, it would be reasonable to add it for all targets
that compiled with toolchain supporting 128-bit accesses.

There's already the option ARCH_SUPPORTS_INT128 that is enabled for
x86_64 in arch/x86/Kconfig and conditionally enabled for arm64 in
arch/arm64/Makefile:
KBUILD_CFLAGS    += $(call cc-ifversion, -ge, 0500, -DCONFIG_ARCH_SUPPORTS_INT128)

It is used in include/linux/math64.h and in lib/ubsan.c, not so wide.

So I find things little messed. Crypto code ignores compilers' ability
to operate with 128-bit numbers. Ubsan and math64 relies on compiler
version (at least for arm64, and I doubt it would work correctly with clang).
And now I introduce HAVE_128BIT_ACCESS with the same meaning for memory
access.

I think it's time to unify 128-bit code:
 - enable CONFIG_ARCH_SUPPORTS_INT128 if compiler supports it, ie check
   it like you do below;
 - declare u128 as structure or typedef depending on ARCH_SUPPORTS_INT128
   in generic include/linux/int128.h, as you suggest here;
 - switch this series to ARCH_SUPPORTS_INT128.

Does it sound reasonable?

The CONFIG_* symbol namespace should not be set dynamically. However, you
can define a symbol with another name, e.g. ARCH_SUPPORTS_INT128 (without
CONFIG_ prefix) in linux/compiler-gcc.h based on the version and BITS_PER_LONG.

         Arnd