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

[Jason Wang]

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.

>     msix_unuse_all_vectors(&proxy->pci_dev);
>
>     for (i = 0; i < VIRTIO_QUEUE_MAX; i++) {
>         proxy->vqs[i].enabled = 0;
>         proxy->vqs[i].num = 0;
>         proxy->vqs[i].desc[0] = proxy->vqs[i].desc[1] = 0;
>         proxy->vqs[i].avail[0] = proxy->vqs[i].avail[1] = 0;
>         proxy->vqs[i].used[0] = proxy->vqs[i].used[1] = 0;
>     }
>
>
> so far so good
>
>     if (pci_is_express(dev)) {
>         pcie_cap_deverr_reset(dev);
>         pcie_cap_lnkctl_reset(dev);
>
>         pci_set_word(dev->config + dev->exp.pm_cap + PCI_PM_CTRL, 0);
>     }
>
> this part is wrong I think, it got here by mistake since the same
> function is used for bus level reset.
>
> Jason, Marcel, any input?

Yes, I think we don't need PCI stuff here. We do virtio reset not pci.

Thanks

>
> --
> MST
>