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Yet, there're limitations for an HWPT-based >>> structure to support some advanced HW-accelerated features, such as CMDQV >>> on NVIDIA Grace, and HW-accelerated vIOMMU on AMD. Even for a multi-IOMMU >>> environment, it is not straightforward for nested HWPTs to share the same >>> parent HWPT (stage-2 IO pagetable), with the HWPT infrastructure alone. >> >> could you elaborate a bit for the last sentence in the above paragraph? > > Stage-2 HWPT/domain on ARM holds a VMID. If we share the parent > domain across IOMMU instances, we'd have to make sure that VMID > is available on all IOMMU instances. There comes the limitation > and potential resource starving, so not ideal. got it. > Baolu told me that Intel may have the same: different domain IDs > on different IOMMUs; multiple IOMMU instances on one chip: > https://lore.kernel.org/linux-iommu/cf4fe15c-8bcb-4132-a1fd-b2c8ddf2731b@linux.intel.com/ > So, I think we are having the same situation here. yes, it's called iommu unit or dmar. A typical Intel server can have multiple iommu units. But like Baolu mentioned in that thread, the intel iommu driver maintains separate domain ID spaces for iommu units, which means a given iommu domain has different DIDs when associated with different iommu units. So intel side is not suffering from this so far. > Adding another vIOMMU wrapper on the other hand can allow us to > allocate different VMIDs/DIDs for different IOMMUs. that looks like to generalize the association of the iommu domain and the iommu units? -- Regards, Yi Liu