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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. > > An ARM SMMU has its own VMID pool as well. The suffering comes > from associating VMIDs to one shared parent S2 domain. Is this because of the VMID is tied with a S2 domain? > Does a DID per S1 nested domain or parent S2? If it is per S2, > I think the same suffering applies when we share the S2 across > IOMMU instances? per S1 I think. The iotlb efficiency is low as S2 caches would be tagged with different DIDs even the page table is the same. :) >>> 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? > > A vIOMMU is a presentation/object of a physical IOMMU instance > in a VM. a slice of a physical IOMMU. is it? and you treat S2 hwpt as a resource of the physical IOMMU as well. > This presentation gives a VMM some capability to take > advantage of some of HW resource of the physical IOMMU: > - a VMID is a small HW reousrce to tag the cache; > - a vIOMMU invalidation allows to access device cache that's > not straightforwardly done via an S1 HWPT invalidation; > - a virtual device presentation of a physical device in a VM, > related to the vIOMMU in the VM, which contains some VM-level > info: virtual device ID, security level (ARM CCA), and etc; > - Non-PRI IRQ forwarding to the guest VM; > - HW-accelerated virtualization resource: vCMDQ, AMD VIOMMU; might be helpful to draw a diagram to show what the vIOMMU obj contains.:) -- Regards, Yi Liu