Re: [PATCH 4/5] virtio: Fail if iommu_platform is requested, but unsupported

[Raphael Norwitz <raphael.norwitz@nutanix.com>]

Got it - thanks for the clarification.

Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>

On Thu, Apr 29, 2021 at 11:34:11AM +0200, Kevin Wolf wrote:
> Am 28.04.2021 um 21:24 hat Raphael Norwitz geschrieben:
> > On Thu, Apr 22, 2021 at 07:02:20PM +0200, Kevin Wolf wrote:
> > > Commit 2943b53f6 (' virtio: force VIRTIO_F_IOMMU_PLATFORM') made sure
> > > that vhost can't just reject VIRTIO_F_IOMMU_PLATFORM when it was
> > > requested. However, just adding it back to the negotiated flags isn't
> > > right either because it promises support to the guest that the device
> > > actually doesn't support. One example of a vhost-user device that
> > > doesn't have support for the flag is the vhost-user-blk export of QEMU.
> > > 
> > > Instead of successfully creating a device that doesn't work, just fail
> > > to plug the device when it doesn't support the feature, but it was
> > > requested. This results in much clearer error messages.
> > > 
> > > Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1935019
> > > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > > ---
> > >  hw/virtio/virtio-bus.c | 5 +++++
> > >  1 file changed, 5 insertions(+)
> > > 
> > > diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c
> > > index d6332d45c3..859978d248 100644
> > > --- a/hw/virtio/virtio-bus.c
> > > +++ b/hw/virtio/virtio-bus.c
> > > @@ -69,6 +69,11 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp)
> > >          return;
> > >      }
> > >
> > 
> > Can you explain this check a little more?
> > 
> > Above we have:
> > bool has_iommu = virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
> 
> If I underdstand the code correctly, at this point this is still the
> unchanged value of the iommu_platform=on|off qdev property as given by
> the user.
> 
> > and then we get the host features from the bckend:
> > vdev->host_features = vdc->get_features(vdev, vdev->host_features
> 
> Yes, and now a flag is only set if the user had requested it and the
> backend also supports it.
> 
> > So as is this is catching the case where vdev->host_features had
> > VIRTIO_F_IOMMU_PLATFORM set before (by default?), but doesn't now that
> > the features have been retrieved? 
> > 
> > Why not just:
> >     if (!virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM)) {
> 
> We don't want to fail if the user hadn't even requested the feature, but
> just if it was requested, but could not be provided.
>
> > > +    if (has_iommu && !virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM)) {
> > > +        error_setg(errp, "iommu_platform=true is not supported by the device");
> > > +        return;
> > > +    }
> > > +
> > >      if (klass->device_plugged != NULL) {
> > >          klass->device_plugged(qbus->parent, &local_err);
> > >      }
> 
> Kevin
>