On Thu, Aug 29, 2019 at 04:48:27PM +0100, Will Deacon wrote:

From: Andrew Murray <redacted>

When building for LSE atomics (CONFIG_ARM64_LSE_ATOMICS), if the hardware
or toolchain doesn't support it the existing code will fallback to ll/sc
atomics. It achieves this by branching from inline assembly to a function
that is built with special compile flags. Further this results in the
clobbering of registers even when the fallback isn't used increasing
register pressure.

Improve this by providing inline implementations of both LSE and
ll/sc and use a static key to select between them, which allows for the
compiler to generate better atomics code. Put the LL/SC fallback atomics
in their own subsection to improve icache performance.

Signed-off-by: Andrew Murray <redacted>
Signed-off-by: Will Deacon <will@kernel.org>

For some reason, this causes a clang built kernel to fail to boot in
QEMU. There are no logs, it just never starts. I am off for the next two
days so I am going to try to look into this but you might have some
immediate ideas.

https://github.com/ClangBuiltLinux/linux/issues/649

There is another weird failure that might be somewhat related but I have
no idea.

https://github.com/ClangBuiltLinux/linux/issues/648

Cheers,
Nathan

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