arch: Enable `FEAT_SB` for supported non-Armv8.5-A platforms

The speculation barrier feature (`FEAT_SB`) was introduced with and
made mandatory in the Armv8.5-A extension. It was retroactively made
optional in prior extensions, but the checks in our code-base do not
reflect that, assuming that it is only available in Armv8.5-A or later.

This change introduces the `ENABLE_FEAT_SB` definition, which derives
support for the `sb` instruction in the assembler from the feature
flags passed to it. Note that we assume that if this feature is enabled
then all the cores in the system support it - enabling speculation
barriers for only a subset of the cores is unsupported.
Signed-off-by: Chris Kay <chris.kay@arm.com>
Change-Id: I978ed38829385b221b10ba56d49b78f4756e20ea

Makefile

@@ -245,6 +245,13 @@ endif	# arch-features
 # Determine if FEAT_RNG is supported
 ENABLE_FEAT_RNG		=	$(if $(findstring rng,${arch-features}),1,0)
 
+# Determine if FEAT_SB is supported
+ENABLE_FEAT_SB		=	$(if $(findstring sb,${arch-features}),1,0)
+
+ifeq "8.5" "$(word 1, $(sort 8.5 $(ARM_ARCH_MAJOR).$(ARM_ARCH_MINOR)))"
+ENABLE_FEAT_SB		= 	1
+endif
+
 ifneq ($(findstring armclang,$(notdir $(CC))),)
 TF_CFLAGS_aarch32	=	-target arm-arm-none-eabi $(march32-directive)
 TF_CFLAGS_aarch64	=	-target aarch64-arm-none-eabi $(march64-directive)
@@ -945,6 +952,7 @@ $(eval $(call assert_booleans,\
         COT_DESC_IN_DTB \
         USE_SP804_TIMER \
         ENABLE_FEAT_RNG \
+        ENABLE_FEAT_SB \
 )))
 
 $(eval $(call assert_numerics,\
@@ -1038,6 +1046,7 @@ $(eval $(call add_defines,\
         COT_DESC_IN_DTB \
         USE_SP804_TIMER \
         ENABLE_FEAT_RNG \
+        ENABLE_FEAT_SB \
 )))
 
 ifeq (${SANITIZE_UB},trap)

include/arch/aarch32/asm_macros.S

@@ -107,12 +107,12 @@
 
 #else
 	/*
-	 * Macro for mitigating against speculative execution beyond ERET.
-	 * If possible use Speculation Barrier instruction defined in ARMv8.5
+	 * Macro for mitigating against speculative execution beyond ERET. Uses the
+	 * speculation barrier instruction introduced by FEAT_SB, if it's enabled.
 	 */
 	.macro exception_return
 	eret
-#if ARM_ARCH_AT_LEAST(8, 5)
+#if ENABLE_FEAT_SB
 	sb
 #else
 	dsb	nsh

include/arch/aarch64/asm_macros.S

@@ -219,12 +219,12 @@
 	.endm
 
 	/*
-	 * Macro for mitigating against speculative execution beyond ERET.
-	 * If possible use Speculation Barrier instruction defined in ARMv8.5
+	 * Macro for mitigating against speculative execution beyond ERET. Uses the
+	 * speculation barrier instruction introduced by FEAT_SB, if it's enabled.
 	 */
 	.macro exception_return
 	eret
-#if ARM_ARCH_AT_LEAST(8, 5)
+#if ENABLE_FEAT_SB
 	sb
 #else
 	dsb	nsh