arm64: errata: Don't enable workarounds for "rare" errata by default

Arm classifies some of its CPU errata as "rare", indicating that the
hardware error is unlikely to occur in practice. Given that the cost of
errata workarounds can often be significant in terms of power and
performance, don't enable workarounds for "rare" errata by default and
update our documentation to reflect that.

Cc: James Morse <james.morse@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20240209183916.25860-1-will@kernel.orgSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

Documentation/arch/arm64/silicon-errata.rst

@@ -35,8 +35,9 @@ can be triggered by Linux).
 For software workarounds that may adversely impact systems unaffected by
 the erratum in question, a Kconfig entry is added under "Kernel
 Features" -> "ARM errata workarounds via the alternatives framework".
-These are enabled by default and patched in at runtime when an affected
-CPU is detected. For less-intrusive workarounds, a Kconfig option is not
+With the exception of workarounds for errata deemed "rare" by Arm, these
+are enabled by default and patched in at runtime when an affected CPU is
+detected. For less-intrusive workarounds, a Kconfig option is not
 available and the code is structured (preferably with a comment) in such
 a way that the erratum will not be hit.
 

arch/arm64/Kconfig

@@ -547,9 +547,8 @@ config ARM64_ERRATUM_832075
 	  If unsure, say Y.
 
 config ARM64_ERRATUM_834220
-	bool "Cortex-A57: 834220: Stage 2 translation fault might be incorrectly reported in presence of a Stage 1 fault"
+	bool "Cortex-A57: 834220: Stage 2 translation fault might be incorrectly reported in presence of a Stage 1 fault (rare)"
 	depends on KVM
-	default y
 	help
 	  This option adds an alternative code sequence to work around ARM
 	  erratum 834220 on Cortex-A57 parts up to r1p2.
@@ -565,7 +564,7 @@ config ARM64_ERRATUM_834220
 	  as it depends on the alternative framework, which will only patch
 	  the kernel if an affected CPU is detected.
 
-	  If unsure, say Y.
+	  If unsure, say N.
 
 config ARM64_ERRATUM_1742098
 	bool "Cortex-A57/A72: 1742098: ELR recorded incorrectly on interrupt taken between cryptographic instructions in a sequence"
@@ -692,8 +691,7 @@ config ARM64_WORKAROUND_REPEAT_TLBI
 	bool
 
 config ARM64_ERRATUM_2441007
-	bool "Cortex-A55: Completion of affected memory accesses might not be guaranteed by completion of a TLBI"
-	default y
+	bool "Cortex-A55: Completion of affected memory accesses might not be guaranteed by completion of a TLBI (rare)"
 	select ARM64_WORKAROUND_REPEAT_TLBI
 	help
 	  This option adds a workaround for ARM Cortex-A55 erratum #2441007.
@@ -706,11 +704,10 @@ config ARM64_ERRATUM_2441007
 	  Work around this by adding the affected CPUs to the list that needs
 	  TLB sequences to be done twice.
 
-	  If unsure, say Y.
+	  If unsure, say N.
 
 config ARM64_ERRATUM_1286807
-	bool "Cortex-A76: Modification of the translation table for a virtual address might lead to read-after-read ordering violation"
-	default y
+	bool "Cortex-A76: Modification of the translation table for a virtual address might lead to read-after-read ordering violation (rare)"
 	select ARM64_WORKAROUND_REPEAT_TLBI
 	help
 	  This option adds a workaround for ARM Cortex-A76 erratum 1286807.
@@ -724,6 +721,8 @@ config ARM64_ERRATUM_1286807
 	  invalidated has been observed by other observers. The
 	  workaround repeats the TLBI+DSB operation.
 
+	  If unsure, say N.
+
 config ARM64_ERRATUM_1463225
 	bool "Cortex-A76: Software Step might prevent interrupt recognition"
 	default y
@@ -743,8 +742,7 @@ config ARM64_ERRATUM_1463225
 	  If unsure, say Y.
 
 config ARM64_ERRATUM_1542419
-	bool "Neoverse-N1: workaround mis-ordering of instruction fetches"
-	default y
+	bool "Neoverse-N1: workaround mis-ordering of instruction fetches (rare)"
 	help
 	  This option adds a workaround for ARM Neoverse-N1 erratum
 	  1542419.
@@ -756,7 +754,7 @@ config ARM64_ERRATUM_1542419
 	  Workaround the issue by hiding the DIC feature from EL0. This
 	  forces user-space to perform cache maintenance.
 
-	  If unsure, say Y.
+	  If unsure, say N.
 
 config ARM64_ERRATUM_1508412
 	bool "Cortex-A77: 1508412: workaround deadlock on sequence of NC/Device load and store exclusive or PAR read"
@@ -931,8 +929,7 @@ config ARM64_ERRATUM_2224489
 	  If unsure, say Y.
 
 config ARM64_ERRATUM_2441009
-	bool "Cortex-A510: Completion of affected memory accesses might not be guaranteed by completion of a TLBI"
-	default y
+	bool "Cortex-A510: Completion of affected memory accesses might not be guaranteed by completion of a TLBI (rare)"
 	select ARM64_WORKAROUND_REPEAT_TLBI
 	help
 	  This option adds a workaround for ARM Cortex-A510 erratum #2441009.
@@ -945,7 +942,7 @@ config ARM64_ERRATUM_2441009
 	  Work around this by adding the affected CPUs to the list that needs
 	  TLB sequences to be done twice.
 
-	  If unsure, say Y.
+	  If unsure, say N.
 
 config ARM64_ERRATUM_2064142
 	bool "Cortex-A510: 2064142: workaround TRBE register writes while disabled"