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

[Robin Murphy]

On 2021-09-07 08:41, Kunkun Jiang wrote:
> 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.

Sure, it's not impossible, there are just a lot of fiddly 
considerations, mostly around how to handle SMMU instances with 
different capabilities. We haven't had a strong need to support this 
case so far, so we've simply been avoiding all that complexity.

Robin.

> Looking forward to your suggestions.😁
>
> Thanks,
> Kunkun Jiang