[Xen-changelog] [xen staging] retpoline: disable jump tables

[p***@xen.org]

commit 2276da0db9b0f111e1a8f41dd707c6bd1dea9a7d
Author: Norbert Manthey <***@amazon.de>
AuthorDate: Wed Nov 21 10:52:05 2018 +0100
Commit: Jan Beulich <***@suse.com>
CommitDate: Wed Nov 21 10:52:05 2018 +0100

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 <***@amazon.de>
Reported-by: Pawel Wieczorkiewicz <***@amazon.de>
Signed-off-by: Norbert Manthey <***@amazon.de>
Acked-by: Jan Beulich <***@suse.com>
---
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

--
generated by git-patchbot for /home/xen/git/xen.git#staging