arm64: Kconfig: Make SME depend on BROKEN for now

Although support for SME was merged in v5.19, we've since uncovered a
number of issues with the implementation, including issues which might
corrupt the FPSIMD/SVE/SME state of arbitrary tasks. While there are
patches to address some of these issues, ongoing review has highlighted
additional functional problems, and more time is necessary to analyse
and fix these.

For now, mark SME as BROKEN in the hope that we can fix things properly
in the near future. As SME is an OPTIONAL part of ARMv9.2+, and there is
very little extant hardware, this should not adversely affect the vast
majority of users.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Cc: stable@vger.kernel.org # 5.19
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20241106164220.2789279-1-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

Author: mrutland-arm

arch/arm64/Kconfig

@@ -2214,6 +2214,7 @@ config ARM64_SME
 	bool "ARM Scalable Matrix Extension support"
 	default y
 	depends on ARM64_SVE
+	depends on BROKEN
 	help
 	  The Scalable Matrix Extension (SME) is an extension to the AArch64
 	  execution state which utilises a substantial subset of the SVE