x86/cmdline: Disable jump tables for cmdline.c

When CONFIG_RETPOLINE is disabled, Clang uses a jump table for the
switch statement in cmdline_find_option (jump tables are disabled when
CONFIG_RETPOLINE is enabled). This function is called very early in boot
from sme_enable() if CONFIG_AMD_MEM_ENCRYPT is enabled. At this time,
the kernel is still executing out of the identity mapping, but the jump
table will contain virtual addresses.

Fix this by disabling jump tables for cmdline.c when AMD_MEM_ENCRYPT is
enabled.
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200903023056.3914690-1-nivedita@alum.mit.edu

arch/x86/lib/Makefile

@@ -24,7 +24,7 @@ ifdef CONFIG_FUNCTION_TRACER
 CFLAGS_REMOVE_cmdline.o = -pg
 endif
 
-CFLAGS_cmdline.o := -fno-stack-protector
+CFLAGS_cmdline.o := -fno-stack-protector -fno-jump-tables
 endif
 
 inat_tables_script = $(srctree)/arch/x86/tools/gen-insn-attr-x86.awk