Just come to mind that there may be a corner case here.
Intel VT-d actually has a "pt" mode which allows device use physical address
even when VT-d is enabled. In kernel, there is a iommu_identity_mapping.
If a device is in this map, then it would use "pt" mode. So that IOMMU driver
would not build second-level page table for it.
Yes, but qemu does not support ECAP_PT now, so guest will still have a page
table in this case.
Back to the virtual IOVA implementation, if an assigned device is in the
iommu_identity_mapping(e.g. VGA controller), it uses GPA directly to do DMA.
So it demands a GPA->HPA mapping in host. However, the iommu->ops.replay
is not able to build it when guest SL page table is empty.
So I think building an entire guest PA->HPA mapping before guest kernel boot
would be recommended. Any thoughts?
We plan to add PT in 2.10, a possible rough idea is disabled iommu dmar
region and use another region without iommu_ops. Then
vfio_listener_region_add() will just do the correct mappings.