[Qemu-devel] [PATCH v2] vhost: Fix aborting if KVM does not support eventfds

[Pavel Fedin <p.fedin@samsung.com>]

If you happen to have a stock kernel of old version, like 3.x, and you
attempt to enable vhost by setting vhost=on, qemu aborts with error:

kvm_mem_ioeventfd_add: error adding ioeventfd: Function not implemented

This patch adds capability check, so that vhost gets disabled instead. A
warning is displayed, explaining the reason:

2015-11-13T08:43:51.146802Z qemu-system-aarch64: KVM does not support eventfd binding
2015-11-13T08:43:51.146915Z qemu-system-aarch64: unable to start vhost net: 38: falling back on userspace virtio

This problem can be observed with libvirt, which checks for /dev/vhost-net
availability and just inserts "vhost=on" automatically in this case.

Signed-off-by: Pavel Fedin <p.fedin@samsung.com>
---
v1 => v2:
- Removed "MMIO" from warning message, because it applies not only to MMIO
- Add note about warning to the commit message
---
 hw/virtio/vhost.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index 1794f0d..50b8171 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -24,6 +24,7 @@
 #include "hw/virtio/virtio-bus.h"
 #include "hw/virtio/virtio-access.h"
 #include "migration/migration.h"
+#include "sysemu/kvm.h"
 
 static struct vhost_log *vhost_log;
 static struct vhost_log *vhost_log_shm;
@@ -1083,6 +1084,11 @@ int vhost_dev_enable_notifiers(struct vhost_dev *hdev, VirtIODevice *vdev)
         r = -ENOSYS;
         goto fail;
     }
+    if (!kvm_eventfds_enabled()) {
+        error_report("KVM does not support eventfd binding");
+        r = -ENOSYS;
+        goto fail;
+    }
 
     for (i = 0; i < hdev->nvqs; ++i) {
         r = k->set_host_notifier(qbus->parent, hdev->vq_index + i, true);
-- 
1.9.5.msysgit.0