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Re: virtio: why no full reset on virtio_set_status 0 ?
From: |
Michael S. Tsirkin |
Subject: |
Re: virtio: why no full reset on virtio_set_status 0 ? |
Date: |
Thu, 28 Jul 2022 07:41:04 -0400 |
On Thu, Jul 28, 2022 at 09:43:56AM +0200, 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?
Well we want to first stop the backend and only then reset our
local state. Seems to make sense ...
> 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,
Not sure I got this part.
> while QEMU is left hanging waiting forever for a response to
> VHOST_USER_GET_VRING_BASE issued as a result of vhost_net_stop.
Ineteresting why doesn't socket close after ovs crash cause the read to fail.
> Just saying, I am having more success with the second ordering, but I am
> still studying, don't have the full picture yet.
>
> Thanks,
>
> Claudio
>
> >
> >> 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
> >>
> >
> >
- Re: virtio: why no full reset on virtio_set_status 0 ?, (continued)
- Re: virtio: why no full reset on virtio_set_status 0 ?, Michael S. Tsirkin, 2022/07/28
- Re: virtio: why no full reset on virtio_set_status 0 ?, Claudio Fontana, 2022/07/29
- Re: virtio: why no full reset on virtio_set_status 0 ?, Michael S. Tsirkin, 2022/07/29
- Re: virtio: why no full reset on virtio_set_status 0 ?, Claudio Fontana, 2022/07/29
- Re: virtio: why no full reset on virtio_set_status 0 ?, Alex Bennée, 2022/07/29
- Re: virtio: why no full reset on virtio_set_status 0 ?, Claudio Fontana, 2022/07/29
- Re: virtio: why no full reset on virtio_set_status 0 ?, Claudio Fontana, 2022/07/31
- Re: virtio: why no full reset on virtio_set_status 0 ?,
Michael S. Tsirkin <=
Re: virtio: why no full reset on virtio_set_status 0 ?, Michael S. Tsirkin, 2022/07/27