x86/clang: add retpoline support

Detect whether the compiler supports clang retpoline option and enable
by default if available, just like it's done for gcc.

Note clang already disables jump tables when retpoline is enabled, so
there's no need to also pass the fno-jump-tables parameter. Also clang
already passes the return address in a register always on amd64, so
there's no need for any equivalent mindirect-branch-register
parameter.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/Kconfig

@@ -33,7 +33,8 @@ config ARCH_DEFCONFIG
 	default "arch/x86/configs/x86_64_defconfig"
 
 config CC_HAS_INDIRECT_THUNK
-	def_bool $(cc-option,-mindirect-branch-register)
+	def_bool $(cc-option,-mindirect-branch-register) || \
+	         $(cc-option,-mretpoline-external-thunk)
 
 config INDIRECT_THUNK
 	def_bool y

xen/arch/x86/arch.mk

@@ -47,6 +47,9 @@ ifeq ($(CONFIG_INDIRECT_THUNK),y)
 CFLAGS-$(CONFIG_CC_IS_GCC) += -mindirect-branch=thunk-extern
 CFLAGS-$(CONFIG_CC_IS_GCC) += -mindirect-branch-register
 CFLAGS-$(CONFIG_CC_IS_GCC) += -fno-jump-tables
+
+# Enable clang retpoline support if available.
+CFLAGS-$(CONFIG_CC_IS_CLANG) += -mretpoline-external-thunk
 endif
 
 # If supported by the compiler, reduce stack alignment to 8 bytes. But allow