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

Pierre Morel
2020-06-29T18:49:53Z

On 2020-06-19 11:20, Cornelia Huck wrote:
> On Thu, 18 Jun 2020 00:29:56 +0200
> Halil Pasic <pasic@linux.ibm.com> wrote:
>
>> On Wed, 17 Jun 2020 12:43:57 +0200
>> Pierre Morel <pmorel@linux.ibm.com> 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@redhat.com>

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