Re: [PATCH] virtio: break and reset virtio devices on device_shutdown()

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

On Fri, Feb 21, 2025 at 09:11:51AM +0800, Jason Wang wrote:
> On Fri, Feb 21, 2025 at 7:42 AM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > Hongyu reported a hang on kexec in a VM. QEMU reported invalid memory
> > accesses during the hang.
> >
> >         Invalid read at addr 0x102877002, size 2, region '(null)', reason: rejected
> >         Invalid write at addr 0x102877A44, size 2, region '(null)', reason: rejected
> >         ...
> >
> > It was traced down to virtio-console. Kexec works fine if virtio-console
> > is not in use.
> >
> > The issue is that virtio-console continues to write to the MMIO even after
> > underlying virtio-pci device is reset.
> 
> Some of my questions were not answered so I need to post them again.
> 
> Do we need to fix vitio-console?

this fixes all devices so no.

> Note that we've already break the device in virtio_pci_remove():
> 
> static void virtio_pci_remove(struct pci_dev *pci_dev)
> {
>   struct virtio_pci_device *vp_dev = pci_get_drvdata(pci_dev);
>         struct device *dev = get_device(&vp_dev->vdev.dev);
> 
>         /*
>          * Device is marked broken on surprise removal so that virtio upper
>          * layers can abort any ongoing operation.
>          */
>         if (!pci_device_is_present(pci_dev))
>                 virtio_break_device(&vp_dev->vdev);
> 
> ...


shutdown path does not invoke remove, and I do not want to,
since that will slow down reboots for no good reason.

> >
> > Additionally, Eric noticed that IOMMUs are reset before devices, if
> > devices are not reset on shutdown they continue to poke at guest memory
> > and get errors from the IOMMU. Some devices get wedged then.
> >
> > The problem can be solved by breaking all virtio devices on virtio
> > bus shutdown, then resetting them.
> >
> > Reported-by: Eric Auger <eauger@redhat.com>
> > Reported-by: Hongyu Ning <hongyu.ning@linux.intel.com>
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >  drivers/virtio/virtio.c | 31 +++++++++++++++++++++++++++++++
> >  1 file changed, 31 insertions(+)
> >
> > diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> > index c1cc1157b380..e5b29520d3b2 100644
> > --- a/drivers/virtio/virtio.c
> > +++ b/drivers/virtio/virtio.c
> > @@ -377,6 +377,36 @@ static void virtio_dev_remove(struct device *_d)
> >         of_node_put(dev->dev.of_node);
> >  }
> >
> > +static void virtio_dev_shutdown(struct device *_d)
> > +{
> > +       struct virtio_device *dev = dev_to_virtio(_d);
> > +       struct virtio_driver *drv = drv_to_virtio(dev->dev.driver);
> > +
> > +       /*
> > +        * Stop accesses to or from the device.
> > +        * We only need to do it if there's a driver - no accesses otherwise.
> > +        */
> > +       if (!drv)
> > +               return;
> > +
> > +       /*
> > +        * Some devices get wedged if you kick them after they are
> > +        * reset. Mark all vqs as broken to make sure we don't.
> > +        */
> > +       virtio_break_device(dev);
> > +       /*
> > +        * The below virtio_synchronize_cbs() guarantees that any interrupt
> > +        * for this line arriving after virtio_synchronize_vqs() has completed
> > +        * is guaranteed to see vq->broken as true.
> > +        */
> > +       virtio_synchronize_cbs(dev);
> 
> This looks like a partial re-introduction of the hardening work, but
> the ccw part is still in-completed e.g the synchronization requires a
> read_lock() and depends on CONFIG_VIRTIO_HARDEN_NOTIFICATION (which is
> marked as broken now).

Hmm I don't understand why we can not just take a subchannel lock.
worth thinking about it. as this doesn't regress i think
I can live with it for now.

> Should it better to
> 
> 1) fix the virtio-console

the problem is not unique to console. it was just observed there.
any device can trigger this.

> 2) simply rest in shutdown

then we are back with devices wedged as they are kicked
while reset.

> 3) wait for the hardening work to be done in the future?
> 
> Thanks

as users are seeing this on x86, I want to fix it.
finding a way to fix virtio_synchronize_cbs seems best.

does not need to block this patch.

> > +       /*
> > +        * As IOMMUs are reset on shutdown, this will block device access to memory.
> > +        * Some devices get wedged if this happens, so reset to make sure it does not.
> > +        */
> > +       dev->config->reset(dev);
> > +}
> > +
> >  static const struct bus_type virtio_bus = {
> >         .name  = "virtio",
> >         .match = virtio_dev_match,
> > @@ -384,6 +414,7 @@ static const struct bus_type virtio_bus = {
> >         .uevent = virtio_uevent,
> >         .probe = virtio_dev_probe,
> >         .remove = virtio_dev_remove,
> > +       .shutdown = virtio_dev_shutdown,
> >  };
> >
> >  int __register_virtio_driver(struct virtio_driver *driver, struct module *owner)
> > --
> > MST
> >