x86, xen: record and display initiator of each multicall when debugging

Store the caller for each multicall so we can report it on failure.
Signed-off-by: NIan Campbell <ian.campbell@citrix.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/xen/multicalls.c

@@ -39,6 +39,7 @@ struct mc_buffer {
 	struct multicall_entry entries[MC_BATCH];
 #if MC_DEBUG
 	struct multicall_entry debug[MC_BATCH];
+	void *caller[MC_BATCH];
 #endif
 	unsigned char args[MC_ARGS];
 	struct callback {
@@ -154,11 +155,12 @@ void xen_mc_flush(void)
 			       ret, smp_processor_id());
 			dump_stack();
 			for (i = 0; i < b->mcidx; i++) {
-				printk(KERN_DEBUG "  call %2d/%d: op=%lu arg=[%lx] result=%ld\n",
+				printk(KERN_DEBUG "  call %2d/%d: op=%lu arg=[%lx] result=%ld\t%pF\n",
 				       i+1, b->mcidx,
 				       b->debug[i].op,
 				       b->debug[i].args[0],
-				       b->entries[i].result);
+				       b->entries[i].result,
+				       b->caller[i]);
 			}
 		}
 #endif
@@ -197,6 +199,9 @@ struct multicall_space __xen_mc_entry(size_t args)
 	}
 
 	ret.mc = &b->entries[b->mcidx];
+#ifdef MC_DEBUG
+	b->caller[b->mcidx] = __builtin_return_address(0);
+#endif
 	b->mcidx++;
 	ret.args = &b->args[argidx];
 	b->argidx = argidx + args;