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Due to a historical oversight, we emit a redundant static branch for each atomic/atomic64 operation when CONFIG_ARM64_LSE_ATOMICS is selected. We can safely remove this, making the kernel Image reasonably smaller. When CONFIG_ARM64_LSE_ATOMICS is selected, every LSE atomic operation has two preceding static branches with the same target, e.g. b f7c <kernel_init_freeable+0xa4> b f7c <kernel_init_freeable+0xa4> mov w0, #0x1 // #1 ldadd w0, w0, [x19] This is because the __lse_ll_sc_body() wrapper uses system_uses_lse_atomics(), which checks both `arm64_const_caps_ready` and `cpu_hwcap_keys[ARM64_HAS_LSE_ATOMICS]`, each of which emits a static branch. This has been the case since commit:addfc38672("arm64: atomics: avoid out-of-line ll/sc atomics") However, there was never a need to check `arm64_const_caps_ready`, which was itself introduced in commit:63a1e1c95e("arm64/cpufeature: don't use mutex in bringup path") ... so that cpus_have_const_cap() could fall back to checking the `cpu_hwcaps` bitmap prior to the static keys for individual caps becoming enabled. As system_uses_lse_atomics() doesn't check `cpu_hwcaps`, and doesn't need to as we can safely use the LL/SC atomics prior to enabling the `ARM64_HAS_LSE_ATOMICS` static key, it doesn't need to check `arm64_const_caps_ready`. This patch removes the `arm64_const_caps_ready` check from system_uses_lse_atomics(). As the arch_atomic_* routines are meant to be safely usable in noinstr code, I've also marked system_uses_lse_atomics() as __always_inline. This results in one fewer static branch per atomic operation, with the prior example becoming: b f78 <kernel_init_freeable+0xa0> mov w0, #0x1 // #1 ldadd w0, w0, [x19] Each static branch consists of the branch itself and an associated __jump_table entry. Removing these has a reasonable impact on the Image size, with a GCC 11.1.0 defconfig v5.17-rc2 Image being reduced by 128KiB: | [mark@lakrids:~/src/linux]% ls -al Image* | -rw-r--r-- 1 mark mark 34619904 Feb 3 18:24 Image.baseline | -rw-r--r-- 1 mark mark 34488832 Feb 3 18:33 Image.onebranch Signed-off-by: Mark Rutland <mark.rutland@arm.com> Cc: Ard Biesheuvel <ardb@kernel.org> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Suzuki Poulose <suzuki.poulose@arm.com> Cc: Will Deacon <will@kernel.org> Link: https://lore.kernel.org/r/20220204104439.270567-1-mark.rutland@arm.com Signed-off-by: Will Deacon <will@kernel.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.