[PATCH] qemu-kvm: fix crash on reboot with vhost-net

["Michael S. Tsirkin" <mst@redhat.com>]

When vhost-net is disabled on reboot, we set msix mask notifier
to NULL to disable further mask/unmask notifications.
Code currently tries to pass this NULL to notifier,
leading to a crash.  The right thing to do is:
- if vector is masked, we don't need to notify backend,
  just disable future notifications
- if vector is unmasked, invoke callback to unassign backend,
  then disable future notifications

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

Note: this patch is for qemu-kvm, the code in question
is not in qemu.git.

 hw/msix.c |   15 ++++++++++++---
 1 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/hw/msix.c b/hw/msix.c
index 3ec8805..43361b5 100644
--- a/hw/msix.c
+++ b/hw/msix.c
@@ -613,9 +613,18 @@ int msix_set_mask_notifier(PCIDevice *dev, unsigned vector, void *opaque)
     if (vector >= dev->msix_entries_nr || !dev->msix_entry_used[vector])
         return 0;
 
-    if (dev->msix_mask_notifier)
-        r = dev->msix_mask_notifier(dev, vector, opaque,
-                                    msix_is_masked(dev, vector));
+    if (dev->msix_mask_notifier && !msix_is_masked(dev, vector)) {
+        /* Switching notifiers while vector is unmasked:
+         * mask the old one, unmask the new one. */
+        if (dev->msix_mask_notifier_opaque[vector]) {
+            r = dev->msix_mask_notifier(dev, vector,
+                                        dev->msix_mask_notifier_opaque[vector],
+                                        1);
+        }
+        if (r >= 0 && opaque) {
+            r = dev->msix_mask_notifier(dev, vector, opaque, 0);
+        }
+    }
     if (r >= 0)
         dev->msix_mask_notifier_opaque[vector] = opaque;
     return r;
-- 
1.7.1.rc1.22.g3163