x86/retpoline/xen: Convert Xen hypercall indirect jumps

Convert indirect call in Xen hypercall to use non-speculative sequence,
when CONFIG_RETPOLINE is enabled.

Signed-off-by: David Woodhouse <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Acked-by: Arjan van de Ven <[e-mail]>
Acked-by: Ingo Molnar <[e-mail]>
Reviewed-by: Juergen Gross <[e-mail]>
Cc: [e-mail]
Cc: Rik van Riel <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Josh Poimboeuf <[e-mail]>
Cc: [e-mail]
Cc: Peter Zijlstra <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Jiri Kosina <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Tim Chen <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Paul Turner <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]

arch/x86/include/asm/xen/hypercall.h

···
 #include <asm/page.h>
 #include <asm/pgtable.h>
 #include <asm/smap.h>
+#include <asm/nospec-branch.h>
 
 #include <xen/interface/xen.h>
 #include <xen/interface/sched.h>
···
 	__HYPERCALL_5ARG(a1, a2, a3, a4, a5);
 
 	stac();
-	asm volatile("call *%[call]"
+	asm volatile(CALL_NOSPEC
 		     : __HYPERCALL_5PARAM
-		     : [call] "a" (&hypercall_page[call])
+		     : [thunk_target] "a" (&hypercall_page[call])
 		     : __HYPERCALL_CLOBBER5);
 	clac();