On Tue, 1 Feb 2022 12:36:25 -0300
Daniel Henrique Barboza <danie...@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
