Re: [Qemu-devel] [2.5 issue] virtio-1 in virtio-net and old vhost

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Tue, 1 Dec 2015 13:10:40 +0100
Cornelia Huck <cornelia.huck@de.ibm.com> wrote:

> On Tue, 1 Dec 2015 11:11:08 +0100
> Cornelia Huck <cornelia.huck@de.ibm.com> wrote:
> 
> > Some of our test folks tried to run a recent-ish qemu (nearly 2.5)
> > combined with an old host kernel (and a virtio-1 capable guest).
> > 
> > In that setup, we had the transport (in that case, virtio-ccw)
> > advertise VERSION_1 as it is a revision 1 device. However, the old
> > vhost driver did not support virtio-1 and therefore cleared the
> > VERSION_1 bit. In the end, qemu did not offer VERSION_1 to the guest
> > for a revision 1 device, which the guest treats as a fatal error.
> > 
> > It looks to me as if virtio-pci has the same problem: The kernel will
> > detect a modern device as by the I/O layout and then barf at the
> > missing VERSION_1 feature bit.
> > 
> > We _could_ make this missing VERSION_1 bit non-fatal in the guest, but
> > that does not fix guests that are already out there.
> > 
> > The problem is that the transport cannot know whether the VERSION_1 bit
> > will be pulled from under it later during device setup: This is only
> > done in the ->get_features() callback when virtio-net will handle the
> > features supported by vhost.
> > 
> > I'm currently lacking a good idea on how to fix this, but I think that
> > is an issue that should be dealt with for 2.5...
> 
> What about the following (completely untested)? Have the transport
> verify that VERSION_1 is still present after get_features. Should do
> for virtio-ccw, but I'm not sure whether virtio-pci can be unrolled in
> that way.

I've confirmed that this fixes an old host kernel + new qemu setup that
exhibits the "guest displeased by missing VERSION_1" syndrome for
virtio-ccw.

> 
> diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
> index fb103b7..87ecbc1 100644
> --- a/hw/s390x/virtio-ccw.c
> +++ b/hw/s390x/virtio-ccw.c
> @@ -1555,6 +1555,16 @@ static void virtio_ccw_device_plugged(DeviceState *d, Error **errp)
>                            d->hotplugged, 1);
>  }
>  
> +static void virtio_ccw_post_plugged(DeviceState *d, Error **errp)
> +{
> +   VirtioCcwDevice *dev = VIRTIO_CCW_DEVICE(d);
> +   VirtIODevice *vdev = virtio_bus_get_device(&dev->bus);
> +
> +   if (!virtio_host_has_feature(vdev, VIRTIO_F_VERSION_1)) {
> +       dev->max_rev = 0;
> +   }
> +}
> +
>  static void virtio_ccw_device_unplugged(DeviceState *d)
>  {
>      VirtioCcwDevice *dev = VIRTIO_CCW_DEVICE(d);
> @@ -1891,6 +1901,7 @@ static void virtio_ccw_bus_class_init(ObjectClass *klass, void *data)
>      k->save_config = virtio_ccw_save_config;
>      k->load_config = virtio_ccw_load_config;
>      k->device_plugged = virtio_ccw_device_plugged;
> +    k->post_plugged = virtio_ccw_post_plugged;
>      k->device_unplugged = virtio_ccw_device_unplugged;
>  }
>  
> diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c
> index febda76..81c7cdd 100644
> --- a/hw/virtio/virtio-bus.c
> +++ b/hw/virtio/virtio-bus.c
> @@ -56,6 +56,9 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp)
>      assert(vdc->get_features != NULL);
>      vdev->host_features = vdc->get_features(vdev, vdev->host_features,
>                                              errp);
> +    if (klass->post_plugged != NULL) {
> +        klass->post_plugged(qbus->parent, errp);
> +    }
>  }
>  
>  /* Reset the virtio_bus */
> diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> index dd48562..06e449c 100644
> --- a/hw/virtio/virtio-pci.c
> +++ b/hw/virtio/virtio-pci.c
> @@ -1741,6 +1741,30 @@ static void virtio_pci_device_plugged(DeviceState *d, Error **errp)
>      virtio_add_feature(&vdev->host_features, VIRTIO_F_BAD_FEATURE);
>  }
>  
> +static void virtio_pci_post_plugged(DeviceState *d, Error **errp)
> +{
> +    VirtIOPCIProxy *proxy = VIRTIO_PCI(d);
> +    bool legacy = !(proxy->flags & VIRTIO_PCI_FLAG_DISABLE_LEGACY);
> +    bool modern = !(proxy->flags & VIRTIO_PCI_FLAG_DISABLE_MODERN);
> +    bool modern_pio = proxy->flags & VIRTIO_PCI_FLAG_MODERN_PIO_NOTIFY;
> +    VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
> +
> +    if (modern && !virtio_host_has_feature(vdev, VIRTIO_F_VERSION_1)) {
> +        if (legacy) {
> +            virtio_pci_modern_mem_region_unmap(proxy, &proxy->common);
> +            virtio_pci_modern_mem_region_unmap(proxy, &proxy->isr);
> +            virtio_pci_modern_mem_region_unmap(proxy, &proxy->device);
> +            virtio_pci_modern_mem_region_unmap(proxy, &proxy->notify);
> +            if (modern_pio) {
> +                virtio_pci_modern_io_region_unmap(proxy, &proxy->notify_pio);
> +            }
> +            proxy->flags |= VIRTIO_PCI_FLAG_DISABLE_MODERN;
> +        } else {
> +            error_setg(errp, "can't fall back to legacy virtio");
> +        }
> +    }
> +}
> +
>  static void virtio_pci_device_unplugged(DeviceState *d)
>  {
>      VirtIOPCIProxy *proxy = VIRTIO_PCI(d);
> @@ -2474,6 +2498,7 @@ static void virtio_pci_bus_class_init(ObjectClass *klass, void *data)
>      k->set_guest_notifiers = virtio_pci_set_guest_notifiers;
>      k->vmstate_change = virtio_pci_vmstate_change;
>      k->device_plugged = virtio_pci_device_plugged;
> +    k->post_plugged = virtio_pci_post_plugged;
>      k->device_unplugged = virtio_pci_device_unplugged;
>      k->query_nvectors = virtio_pci_query_nvectors;
>  }
> diff --git a/include/hw/virtio/virtio-bus.h b/include/hw/virtio/virtio-bus.h
> index 6c3d4cb..ff0a3b0 100644
> --- a/include/hw/virtio/virtio-bus.h
> +++ b/include/hw/virtio/virtio-bus.h
> @@ -59,6 +59,7 @@ typedef struct VirtioBusClass {
>       * This is called by virtio-bus just after the device is plugged.
>       */
>      void (*device_plugged)(DeviceState *d, Error **errp);
> +    void (*post_plugged)(DeviceState *d, Error **errp);
>      /*
>       * transport independent exit function.
>       * This is called by virtio-bus just before the device is unplugged.