[question] Assign multiple devices from different SMMUs to a arm_smmu_domain

[Kunkun Jiang]

Hi all,

I am working on VFIO DMA dirty pages tracking based on ARM SMMU HTTU,
and have done a lot of testing.In the test, I found a problem that 
greatly affects
performance of VFIO DMA dirty pages tracking.

According to the current arm-smmu-v3 driver, multiple VFIO pass-through
device comes from different SMMUs will be assigned to different
arm_smmu_domain. It will create page table for each arm_smmu_domain,
even though these page tables are exactly the same. Bacause dirty pages
tracking needs to traverse the page table, multiple page tables will make
performance worse.

I learned the ARM SMMUv3 spec and had some exchanges with my colleagues
who work on SMMU hardware. I did not find the restriction that multiple
SMMUs cannot share the same page table. We migth be able to do this like
x86 IOMMU. If I have missed something, please point it out.

Looking forward to your suggestions.😁

Thanks,
Kunkun Jiang