ANDROID: Setting up GS before calling __restore_processor_state.

Android Common Kernel compiled by clang with Shadow Call Stack will
use GS segment. However, __restore_processor_state is called when
system wakes up from S3 and at the moment GS is not restored yet.

This is a hack by copying a small code snippet(setting gs base) from
__restore_processor_state to restore_processor_state. It prepares GS
before __restore_processor_state is called. At the same time,
restore_processor_state is still small enough so that SCS is not on,
as SCS seems to be on only for large functions.

Bug: 166163480
Change-Id: I3bfe4ac61dee876da57de6578c9a7f01431a1743
Signed-off-by: Haitao Shan <hshan@google.com>

arch/x86/power/cpu.c

@@ -274,6 +274,19 @@ static void notrace __restore_processor_state(struct saved_context *ctxt)
 /* Needed by apm.c */
 void notrace restore_processor_state(void)
 {
+#ifdef __clang__
+	// The following code snippet is copied from __restore_processor_state.
+	// Its purpose is to prepare GS segment before the function is called.
+	// Since the function is compiled with SCS on, it will use GS at its
+	// entry.
+	// TODO: Hack to be removed later when compiler bug is fixed.
+#ifdef CONFIG_X86_64
+	wrmsrl(MSR_GS_BASE, saved_context.kernelmode_gs_base);
+#else
+	loadsegment(fs, __KERNEL_PERCPU);
+	loadsegment(gs, __KERNEL_STACK_CANARY);
+#endif
+#endif
 	__restore_processor_state(&saved_context);
 }
 #ifdef CONFIG_X86_32