x86/xen: Move hypercall_page to top of the file

Because hypercall_page is page-aligned, the assembler inexplicably adds
an unreachable jump from after the end of the previous code to the
beginning of hypercall_page.

That confuses objtool, understandably.  It also creates significant text
fragmentation.  As a result, much of the object file is wasted text
(nops).

Move hypercall_page to the beginning of the file to both prevent the
text fragmentation and avoid the dead jump instruction.

$ size /tmp/head_64.before.o /tmp/head_64.after.o
   text	   data	    bss	    dec	    hex	filename
  10924	 307252	   4096	 322272	  4eae0	/tmp/head_64.before.o
   6823	 307252	   4096	 318171	  4dadb	/tmp/head_64.after.o

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lkml.kernel.org/r/20210820193107.omvshmsqbpxufzkc@treble

arch/x86/xen/xen-head.S

@@ -20,6 +20,23 @@
 #include <xen/interface/xen-mca.h>
 #include <asm/xen/interface.h>
 
+.pushsection .text
+	.balign PAGE_SIZE
+SYM_CODE_START(hypercall_page)
+	.rept (PAGE_SIZE / 32)
+		UNWIND_HINT_FUNC
+		.skip 31, 0x90
+		ret
+	.endr
+
+#define HYPERCALL(n) \
+	.equ xen_hypercall_##n, hypercall_page + __HYPERVISOR_##n * 32; \
+	.type xen_hypercall_##n, @function; .size xen_hypercall_##n, 32
+#include <asm/xen-hypercalls.h>
+#undef HYPERCALL
+SYM_CODE_END(hypercall_page)
+.popsection
+
 #ifdef CONFIG_XEN_PV
 	__INIT
 SYM_CODE_START(startup_xen)
@@ -64,23 +81,6 @@ SYM_CODE_END(asm_cpu_bringup_and_idle)
 #endif
 #endif
 
-.pushsection .text
-	.balign PAGE_SIZE
-SYM_CODE_START(hypercall_page)
-	.rept (PAGE_SIZE / 32)
-		UNWIND_HINT_FUNC
-		.skip 31, 0x90
-		ret
-	.endr
-
-#define HYPERCALL(n) \
-	.equ xen_hypercall_##n, hypercall_page + __HYPERVISOR_##n * 32; \
-	.type xen_hypercall_##n, @function; .size xen_hypercall_##n, 32
-#include <asm/xen-hypercalls.h>
-#undef HYPERCALL
-SYM_CODE_END(hypercall_page)
-.popsection
-
 	ELFNOTE(Xen, XEN_ELFNOTE_GUEST_OS,       .asciz "linux")
 	ELFNOTE(Xen, XEN_ELFNOTE_GUEST_VERSION,  .asciz "2.6")
 	ELFNOTE(Xen, XEN_ELFNOTE_XEN_VERSION,    .asciz "xen-3.0")