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.