[PATCH] xen/x86: add diagnostic printout to xen_mc_flush() in case of error

[Juergen Gross]

Failure of an element of a Xen multicall is signalled via a WARN()
only unless the kernel is compiled with MC_DEBUG. It is impossible to
know which element failed and why it did so.

Change that by printing the related information even without MC_DEBUG,
even if maybe in some limited form (e.g. without information which
caller produced the failing element).

Move the printing out of the switch statement in order to have the
same information for a single call.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 arch/x86/xen/multicalls.c | 35 ++++++++++++++++++++---------------
 1 file changed, 20 insertions(+), 15 deletions(-)

diff --git a/arch/x86/xen/multicalls.c b/arch/x86/xen/multicalls.c
index 2bce7958ce8b..0766a08bdf45 100644
--- a/arch/x86/xen/multicalls.c
+++ b/arch/x86/xen/multicalls.c
@@ -69,6 +69,11 @@ void xen_mc_flush(void)
 
 	trace_xen_mc_flush(b->mcidx, b->argidx, b->cbidx);
 
+#if MC_DEBUG
+	memcpy(b->debug, b->entries,
+	       b->mcidx * sizeof(struct multicall_entry));
+#endif
+
 	switch (b->mcidx) {
 	case 0:
 		/* no-op */
@@ -87,32 +92,34 @@ void xen_mc_flush(void)
 		break;
 
 	default:
-#if MC_DEBUG
-		memcpy(b->debug, b->entries,
-		       b->mcidx * sizeof(struct multicall_entry));
-#endif
-
 		if (HYPERVISOR_multicall(b->entries, b->mcidx) != 0)
 			BUG();
 		for (i = 0; i < b->mcidx; i++)
 			if (b->entries[i].result < 0)
 				ret++;
+	}
 
+	if (WARN_ON(ret)) {
+		pr_err("%d of %d multicall(s) failed: cpu %d\n",
+		       ret, b->mcidx, smp_processor_id());
+		for (i = 0; i < b->mcidx; i++) {
+			if (b->entries[i].result < 0) {
 #if MC_DEBUG
-		if (ret) {
-			printk(KERN_ERR "%d multicall(s) failed: cpu %d\n",
-			       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\t%pF\n",
-				       i+1, b->mcidx,
+				pr_err("  call %2d: op=%lu arg=[%lx] result=%ld\t%pF\n",
+				       i + 1,
 				       b->debug[i].op,
 				       b->debug[i].args[0],
 				       b->entries[i].result,
 				       b->caller[i]);
+#else
+				pr_err("  call %2d: op=%lu arg=[%lx] result=%ld\n",
+				       i + 1,
+				       b->entries[i].op,
+				       b->entries[i].args[0],
+				       b->entries[i].result);
+#endif
 			}
 		}
-#endif
 	}
 
 	b->mcidx = 0;
@@ -126,8 +133,6 @@ void xen_mc_flush(void)
 	b->cbidx = 0;
 
 	local_irq_restore(flags);
-
-	WARN_ON(ret);
 }
 
 struct multicall_space __xen_mc_entry(size_t args)
-- 
2.16.4