Re: [Qemu-devel] [PATCH] vhost-scsi: Update 'ioeventfd_started' with host notifiers

[Paolo Bonzini]

On 07/11/2016 18:23, Felipe Franciosi wrote:
> Following the recent refactor of virtio notfiers [1], more specifically
> the patch that uses virtio_bus_set_host_notifier [2] by default, core
> virtio code requires 'ioeventfd_started' to be set to true/false when
> the host notifiers are configured. Since vhost-scsi uses the legacy
> interface, this value is not updated.
> 
> When booting a guest with a vhost-scsi backend controller, SeaBIOS will
> initially configure the device which sets all notifiers. The guest will
> continue to boot fine until the kernel virtio-scsi module reinitialises
> the device causing a stop followed by another start. Since
> ioeventfd_started was never set to true, the 'stop' operation triggered
> by virtio_bus_set_host_notifier() will not result in a call to
> virtio_pci_ioeventfd_assign(assign=false). This leaves the memory
> regions with stale notifiers and results on the next start triggering
> the following assertion:
> 
>   kvm_mem_ioeventfd_add: error adding ioeventfd: File exists
>   Aborted
> 
> This patch updates ioeventfd_started whenever the notifiers are set or
> cleared, fixing this issue.
> 
> Signed-off-by: Felipe Franciosi <address@hidden>
> 
> [1] http://lists.nongnu.org/archive/html/qemu-devel/2016-10/msg07748.html
> [2] http://lists.nongnu.org/archive/html/qemu-devel/2016-10/msg07760.html
> ---
>  hw/scsi/vhost-scsi.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/hw/scsi/vhost-scsi.c b/hw/scsi/vhost-scsi.c
> index 5b26946..1c6e6d4 100644
> --- a/hw/scsi/vhost-scsi.c
> +++ b/hw/scsi/vhost-scsi.c
> @@ -95,6 +95,7 @@ static int vhost_scsi_start(VHostSCSI *s)
>      if (ret < 0) {
>          return ret;
>      }
> +    VIRTIO_BUS(qbus)->ioeventfd_started = true;
>  
>      s->dev.acked_features = vdev->guest_features;
>      ret = vhost_dev_start(&s->dev, vdev);
> @@ -152,6 +153,7 @@ static void vhost_scsi_stop(VHostSCSI *s)
>      vhost_scsi_clear_endpoint(s);
>      vhost_dev_stop(&s->dev, vdev);
>      vhost_dev_disable_notifiers(&s->dev, vdev);
> +    VIRTIO_BUS(qbus)->ioeventfd_started = false;
>  }
>  
>  static uint64_t vhost_scsi_get_features(VirtIODevice *vdev,
> 

While a bit hacky, at least for 2.8 the idea should be fine.  Only it
has to be done in vhost_dev_enable_notifiers and
vhost_dev_disable_notifiers, close to the calls to
virtio_device_stop_ioeventfd (i.e., set bus->ioeventfd_started after the
call) and virtio_device_start_ioeventfd (clear it before the call).

I've now worked through a fix that uses start_ioeventfd/stop_ioeventfd,
and it does comes out nice (29 lines added, 65 removed :)), but it isn't
bisectable (so it has to be squashed into a single patch) and does not
cover vhost-net yet.  So let's go with your patch for now.

I'll send the vm_running fix separately, since that one is a real bugfix.

Paolo