Re: [PATCH v3 1/1] s390: virtio: let arch accept devices without IOMMU feature

[Pierre Morel]

On 2020-06-19 11:20, Cornelia Huck wrote:
> On Thu, 18 Jun 2020 00:29:56 +0200
> Halil Pasic <pasic@xxxxxxxxxxxxx> wrote:
>
> > On Wed, 17 Jun 2020 12:43:57 +0200
> > Pierre Morel <pmorel@xxxxxxxxxxxxx> wrote:
...
> > But since this can be rewritten any time, let's go with the option
> > people already agree with, instead of more discussion.
>
> Yes, there's nothing wrong with the patch as-is.
>
> Acked-by: Cornelia Huck <cohuck@xxxxxxxxxx>

Thanks,


> Which tree should this go through? Virtio? s390? >
> > Just another question. Do we want this backported? Do we need cc stable?
>
> It does change behaviour of virtio-ccw devices; but then, it only
> fences off configurations that would not have worked anyway.
> Distributions should probably pick this; but I do not consider it
> strictly a "fix" (more a mitigation for broken configurations), so I'm
> not sure whether stable applies.
>
> > [..]
> >
> >
> > >   int virtio_finalize_features(struct virtio_device *dev)
> > >   {
> > >   	int ret = dev->config->finalize_features(dev);
> > > @@ -179,6 +194,13 @@ int virtio_finalize_features(struct virtio_device *dev)
> > >   	if (!virtio_has_feature(dev, VIRTIO_F_VERSION_1))
> > >   		return 0;
> > >   
> > > +	if (arch_needs_virtio_iommu_platform(dev) &&
> > > +		!virtio_has_feature(dev, VIRTIO_F_IOMMU_PLATFORM)) {
> > > +		dev_warn(&dev->dev,
> > > +			 "virtio: device must provide VIRTIO_F_IOMMU_PLATFORM\n");
> >
> > I'm not sure, divulging the current Linux name of this feature bit is a
> > good idea, but if everybody else is fine with this, I don't care that
>
> Not sure if that feature name will ever change, as it is exported in
> headers. At most, we might want to add the new ACCESS_PLATFORM define
> and keep the old one, but that would still mean some churn.
>
> > much. An alternative would be:
> > "virtio: device falsely claims to have full access to the memory,
> > aborting the device"
>
> "virtio: device does not work with limited memory access" ?
>
> But no issue with keeping the current message.

If it is OK, I would like to specify that the arch is responsible to 
accept or not the device.
The reason why the device is not accepted without IOMMU_PLATFORM is arch 
specific.

Regards,
Pierre

--
Pierre Morel
IBM Lab Boeblingen