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
diff --git a/include/arch/aarch64/asm_macros.S b/include/arch/aarch64/asm_macros.S
index cbb9f0b..464c05b 100644
--- a/include/arch/aarch64/asm_macros.S
+++ b/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