From 2276da0db9b0f111e1a8f41dd707c6bd1dea9a7d Mon Sep 17 00:00:00 2001 From: Norbert Manthey Date: Wed, 21 Nov 2018 10:52:05 +0100 Subject: [PATCH] retpoline: disable jump tables To mitigate Spectre v2, Xen has been fixed with a software fix, namely using retpoline sequences generated by the compiler. This way, indirect branches are protected against the attack. However, the retpoline sequence comes with a slow down. To make up for this, we propose to avoid jump tables in the first place. Without the retpoline sequences, this code would be less efficient. However, when retpoline is enabled, this actually results in a slight performance improvement. This change might become irrelevant once the compiler starts avoiding jump tables in case retpolines are used: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86952 Reported-by: Julian Stecklina Reported-by: Pawel Wieczorkiewicz Signed-off-by: Norbert Manthey Acked-by: Jan Beulich --- xen/arch/x86/Rules.mk | 1 + 1 file changed, 1 insertion(+) diff --git a/xen/arch/x86/Rules.mk b/xen/arch/x86/Rules.mk index cc301cdc5b..3f2687bbe4 100644 --- a/xen/arch/x86/Rules.mk +++ b/xen/arch/x86/Rules.mk @@ -48,6 +48,7 @@ endif ifneq ($(call cc-option,$(CC),-mindirect-branch-register,n),n) CFLAGS += -mindirect-branch=thunk-extern -mindirect-branch-register CFLAGS += -DCONFIG_INDIRECT_THUNK +CFLAGS += -fno-jump-tables export CONFIG_INDIRECT_THUNK=y endif -- 2.30.2