Re: [PATCH v3 1/1] virtio: fix the condition for iommu_platform not supported

[Halil Pasic <pasic@linux.ibm.com>]

On Tue, 1 Feb 2022 12:36:25 -0300
Daniel Henrique Barboza <danielhb@linux.ibm.com> wrote:

> > +    vdev_has_iommu = virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
> >       if (klass->get_dma_as != NULL && has_iommu) {
> >           virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM);
> >           vdev->dma_as = klass->get_dma_as(qbus->parent);
> > +        if (!vdev_has_iommu && vdev->dma_as != &address_space_memory) {
> > +            error_setg(errp,
> > +                       "iommu_platform=true is not supported by the device");
> > +        }  
> 
> 
> >       } else {
> >           vdev->dma_as = &address_space_memory;
> >       }  
> 
> 
> I struggled to understand what this 'else' clause was doing and I assumed that it was
> wrong. Searching through the ML I learned that this 'else' clause is intended to handle
> legacy virtio devices that doesn't support the DMA API (introduced in 8607f5c3072caeebb)
> and thus shouldn't set  VIRTIO_F_IOMMU_PLATFORM.
> 
> 
> My suggestion, if a v4 is required for any other reason, is to add a small comment in this
> 'else' clause explaining that this is the legacy virtio devices condition and those devices
> don't set F_IOMMU_PLATFORM. This would make the code easier to read for a virtio casual like
> myself.

I do not agree that this is about legacy virtio. In my understanding
virtio-ccw simply does not need translation because CCW devices use
guest physical addresses as per architecture. It may be considered
legacy stuff form PCI perspective, but I don't think it is legacy
in general.

And there is a good reason for virtio-ccw devices to use
F_IOMMU_PLATFORM (secure execution).

Other opinions?

Regards,
Halil