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.fe...@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