From: Juergen Gross <jgross@suse.com>
Date: Fri, 23 Nov 2018 17:24:51 +0100
Patch-mainline: 4.20-rc5
Git-commit: a7b403104e17209ea71eea59d4a71bf9e0d8cb83
References: bnc#1116183
Subject: [PATCH] xen/x86: add diagnostic printout to xen_mc_flush() in case of
 error

Failure of an element of a Xen multicall is signalled via a WARN()
only if 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@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
---
 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