[PATCH v4] arm64: errata: Work around AmpereOne's erratum AC04_CPU_23

[Mark Brown]

On Tue, May 13, 2025 at 11:45:14AM -0700, D Scott Phillips wrote:
> On AmpereOne AC04, updates to HCR_EL2 can rarely corrupt simultaneous
> translations for data addresses initiated by load/store instructions.
> Only instruction initiated translations are vulnerable, not translations
> from prefetches for example. A DSB before the store to HCR_EL2 is
> sufficient to prevent older instructions from hitting the window for
> corruption, and an ISB after is sufficient to prevent younger
> instructions from hitting the window for corruption.

This patch, which is in -next as fed55f49fad181be9dfb93c0, breaks the
build of at least the vDSO selftests:

$ make -C tools/testing/selftests ARCH=arm64 LLVM=1 TARGETS=vDSO

  CC       vdso_test_chacha
In file included from vgetrandom-chacha.S:9:
In file included from ./../../../../arch/arm64/kernel/vdso/vgetrandom-chacha.S:5:
In file included from /home/broonie/git/bisect/tools/testing/selftests/../../../arch/arm64/include/asm/assembler.h:21:
In file included from /home/broonie/git/bisect/tools/testing/selftests/../../../arch/arm64/include/asm/cpufeature.h:13:
/home/broonie/git/bisect/tools/testing/selftests/../../../arch/arm64/include/asm/sysreg.h:1097:5: error: function-like macro 'IS_ENABLED' is not defined
 1097 | #if IS_ENABLED(CONFIG_AMPERE_ERRATUM_AC04_CPU_23)
      |     ^
1 error generated.
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