Re: virtio: why no full reset on virtio_set_status 0 ?

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

On Thu, Jul 28, 2022 at 11:09:15AM +0200, Claudio Fontana wrote:
> On 7/28/22 09:43, Claudio Fontana wrote:
> > On 7/28/22 03:27, Jason Wang wrote:
> >> On Wed, Jul 27, 2022 at 11:32 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >>>
> >>> On Wed, Jul 27, 2022 at 12:51:31PM +0200, Claudio Fontana wrote:
> >>>> Hi Michael and all,
> >>>>
> >>>> I have started researching a qemu / ovs / dpdk bug:
> >>>>
> >>>> https://inbox.dpdk.org/dev/322122fb-619d-96f6-5c3e-9eabdbf3819a@redhat.com/T/
> >>>>
> >>>> that seems to be affecting multiple parties in the telco space,
> >>>>
> >>>> and during this process I noticed that qemu/hw/virtio/virtio.c does not do a full virtio reset
> >>>> in virtio_set_status, when receiving a status value of 0.
> >>>>
> >>>> It seems it has always been this way, so I am clearly missing / forgetting something basic,
> >>>>
> >>>> I checked the virtio spec at https://docs.oasis-open.org/
> >>>>
> >>>> and from:
> >>>>
> >>>> "
> >>>> 4.1.4.3 Common configuration structure layout
> >>>>
> >>>> device_status
> >>>> The driver writes the device status here (see 2.1). Writing 0 into this field resets the device.
> >>>>
> >>>> "
> >>>>
> >>>> and
> >>>>
> >>>> "
> >>>> 2.4.1 Device Requirements: Device Reset
> >>>> A device MUST reinitialize device status to 0 after receiving a reset.
> >>>> "
> >>>>
> >>>> I would conclude that in virtio.c::virtio_set_status we should unconditionally do a full virtio_reset.
> >>>>
> >>>> Instead, we have just the check:
> >>>>
> >>>>     if ((vdev->status & VIRTIO_CONFIG_S_DRIVER_OK) !=
> >>>>         (val & VIRTIO_CONFIG_S_DRIVER_OK)) {
> >>>>         virtio_set_started(vdev, val & VIRTIO_CONFIG_S_DRIVER_OK);
> >>>>     }
> >>>>
> >>>> which just sets the started field,
> >>>>
> >>>> and then we have the call to the virtio device class set_status (virtio_net...),
> >>>> but the VirtioDevice is not fully reset, as per the virtio_reset() call we are missing:
> >>>>
> >>>> "
> >>>>     vdev->start_on_kick = false;
> >>>>     vdev->started = false;
> >>>>     vdev->broken = false;
> >>>>     vdev->guest_features = 0;
> >>>>     vdev->queue_sel = 0;
> >>>>     vdev->status = 0;
> >>>>     vdev->disabled = false;
> >>>>     qatomic_set(&vdev->isr, 0);
> >>>>     vdev->config_vector = VIRTIO_NO_VECTOR;
> >>>>     virtio_notify_vector(vdev, vdev->config_vector);
> >>>>
> >>>>     for(i = 0; i < VIRTIO_QUEUE_MAX; i++) {
> >>>>         ... initialize vdev->vq[i] ...
> >>>>     }
> >>>> "
> >>>>
> >>>> Doing a full reset seems to fix the problem for me, so I can send tentative patches if necessary,
> >>>> but what am I missing here?
> >>>>
> >>>> Thanks,
> >>>>
> >>>> Claudio
> >>>>
> >>>> --
> >>>> Claudio Fontana
> >>>> Engineering Manager Virtualization, SUSE Labs Core
> >>>>
> >>>> SUSE Software Solutions Italy Srl
> >>>
> >>>
> >>> So for example for pci:
> >>>
> >>>     case VIRTIO_PCI_STATUS:
> >>>
> >>>
> >>>         ....
> >>>
> >>>         if (vdev->status == 0) {
> >>>             virtio_pci_reset(DEVICE(proxy));
> >>>         }
> >>>
> >>> which I suspect is a bug because:
> >>>
> >>> static void virtio_pci_reset(DeviceState *qdev)
> >>> {
> >>>     VirtIOPCIProxy *proxy = VIRTIO_PCI(qdev);
> >>>     VirtioBusState *bus = VIRTIO_BUS(&proxy->bus);
> >>>     PCIDevice *dev = PCI_DEVICE(qdev);
> >>>     int i;
> >>>
> >>>     virtio_bus_reset(bus);
> >>
> >> Note that we do virtio_reset() here.
> > 
> > 
> > Yes, thank you, I completely overlooked it, I noticed this in Michael's response as well.
> > 
> > However we end up with multiple calls to k->set_status, one from the virtio_set_status call,
> > and one from the virtio_bus_reset(), which is probably something we don't want.
> > 
> > All in all it is not clear what the meaning of virtio_set_status is supposed to be I think,
> > and I wonder what the assumptions are among all the callers.
> > If it is supposed to be an implementation of the virtio standard field as described, I think we should do the reset right then and there,
> > but maybe the true meaning of the function is another one I couldn't understand, since _some_ of the cases are processes there.
> > 
> > And there is a question about ordering:
> > 
> > in virtio_pci we end up calling virtio_set_status(0), which gets us k->set_status(vdev, 0), which lands in virtio_net_set_status(0) and virtio_net_vhost_status,
> > which causes a vhost_net_stop().
> > 
> > Should we instead land in virtio_net_reset() first, by doing a virtio reset earlier when detecting a 0 value from the driver?
> > 
> > in the scenario I am looking at (with vhost-user, ovs/dpdk, and a guest testpmd application),
> > the guest application goes away without any chance to signal (kill -9), then gets immediately restarted and does a write of 0 to status, while qemu and ovs still hold the state for the device.
> > 
> > As QEMU lands in vhost_net_stop(), it seems to cause a chain of events that crash ovs which is trying to read an rx burst from the queue,
> > while QEMU is left hanging waiting forever for a response to VHOST_USER_GET_VRING_BASE issued as a result of vhost_net_stop.
> > 
> > Just saying, I am having more success with the second ordering, but I am still studying, don't have the full picture yet.
> 
> 
> Currently I'm doing (on top of Michael's patch) the following which seems to be working
> (but of course this does not even being to look at the other transports, architectures etc),
> just an idea to share:
> 
> ---
>  hw/virtio/virtio-pci.c | 7 ++++---
>  hw/virtio/virtio.c     | 7 ++++++-
>  2 files changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> index 3189ec014d..3cbfa3ce3a 100644
> --- a/hw/virtio/virtio-pci.c
> +++ b/hw/virtio/virtio-pci.c
> @@ -312,6 +312,7 @@ static void virtio_ioport_write(void *opaque, uint32_t addr, uint32_t val)
>      case VIRTIO_PCI_QUEUE_PFN:
>          pa = (hwaddr)val << VIRTIO_PCI_QUEUE_ADDR_SHIFT;
>          if (pa == 0) {
> +            virtio_bus_reset(&proxy->bus);
>              virtio_pci_reset(DEVICE(proxy));
>          }
>          else
> @@ -1941,11 +1942,8 @@ static void virtio_pci_exit(PCIDevice *pci_dev)
>  static void virtio_pci_reset(DeviceState *qdev)
>  {
>      VirtIOPCIProxy *proxy = VIRTIO_PCI(qdev);
> -    VirtioBusState *bus = VIRTIO_BUS(&proxy->bus);
> -    PCIDevice *dev = PCI_DEVICE(qdev);
>      int i;
>  
> -    virtio_bus_reset(bus);
>      msix_unuse_all_vectors(&proxy->pci_dev);
>  
>      for (i = 0; i < VIRTIO_QUEUE_MAX; i++) {
> @@ -1960,7 +1958,10 @@ static void virtio_pci_reset(DeviceState *qdev)
>  static void virtio_pci_bus_reset(DeviceState *qdev)
>  {
>      PCIDevice *dev = PCI_DEVICE(qdev);
> +    VirtIOPCIProxy *proxy = VIRTIO_PCI(qdev);
> +    VirtioBusState *bus = VIRTIO_BUS(&proxy->bus);
>  
> +    virtio_bus_reset(bus);
>      virtio_pci_reset(qdev);
>  
>      if (pci_is_express(dev)) {
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 5d607aeaa0..da58ca6f86 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -1977,6 +1977,12 @@ int virtio_set_status(VirtIODevice *vdev, uint8_t val)
>      VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
>      trace_virtio_set_status(vdev, val);
>  
> +    if (val == 0) {
> +        VirtioBusState *bus = VIRTIO_BUS(qdev_get_parent_bus(DEVICE(vdev)));
> +        virtio_bus_reset(bus);
> +        return 0;
> +    }
> +
>      if (virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1)) {
>          if (!(vdev->status & VIRTIO_CONFIG_S_FEATURES_OK) &&
>              val & VIRTIO_CONFIG_S_FEATURES_OK) {
> @@ -2025,7 +2031,6 @@ void virtio_reset(void *opaque)
>      VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
>      int i;
>  
> -    virtio_set_status(vdev, 0);
>      if (current_cpu) {
>          /* Guest initiated reset */
>          vdev->device_endian = virtio_current_cpu_endian();
> -- 
> 2.26.2

As you say this is incomplete ... bout could you share a bit more
of what issue does this address?

-- 
MST