[xen staging] x86/clang: add retpoline support

[patchbot@xxxxxxx]

commit 9412486707f8f1ca2eb31c2ef330c5e39c0a2f30
Author:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
AuthorDate: Fri Feb 18 15:34:15 2022 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Mon Feb 21 18:17:56 2022 +0000

    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@xxxxxxxxxx>
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/Kconfig | 3 ++-
 xen/arch/x86/arch.mk | 3 +++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/Kconfig b/xen/arch/x86/Kconfig
index fe89fa7274..1465874097 100644
--- a/xen/arch/x86/Kconfig
+++ b/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
diff --git a/xen/arch/x86/arch.mk b/xen/arch/x86/arch.mk
index 15d0cbe487..edfc043dbb 100644
--- a/xen/arch/x86/arch.mk
+++ b/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
--
generated by git-patchbot for /home/xen/git/xen.git#staging