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Tsirkin" , Cornelia Huck , qemu-stable@nongnu.org, qemu-devel@nongnu.org, Halil Pasic , Jakob Naucke Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Tue, 1 Feb 2022 12:36:25 -0300 Daniel Henrique Barboza 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