Re: Multiple vIOMMU instance support in QEMU?

[Nicolin Chen]

Hi Peter,

Eric previously mentioned that you might not like the idea.
Before we start this big effort, would it possible for you
to comment a word or two on this topic?

Thanks!

On Mon, Apr 24, 2023 at 04:42:57PM -0700, Nicolin Chen wrote:
> Hi all,
> 
> (Please feel free to include related folks into this thread.)
> 
> In light of an ongoing nested-IOMMU support effort via IOMMUFD, we
> would likely have a need of a multi-vIOMMU support in QEMU, or more
> specificly a multi-vSMMU support for an underlying HW that has multi
> physical SMMUs. This would be used in the following use cases.
>  1) Multiple physical SMMUs with different feature bits so that one
>     vSMMU enabling a nesting configuration cannot reflect properly.
>  2) NVIDIA Grace CPU has a VCMDQ HW extension for SMMU CMDQ. Every
>     VCMDQ HW has an MMIO region (CONS and PROD indexes) that should
>     be exposed to a VM, so that a hypervisor can avoid trappings by
>     using this HW accelerator for performance. However, one single
>     vSMMU cannot mmap multiple MMIO regions from multiple pSMMUs.
>  3) With the latest iommufd design, a single vIOMMU model shares the
>     same stage-2 HW pagetable across all physical SMMUs with a shared
>     VMID. Then a stage-1 pagetable invalidation (for one device) at
>     the vSMMU would have to be broadcasted to all the SMMU instances,
>     which would hurt the overall performance.
> 
> I previously discussed with Eric this topic in a private email. Eric
> felt the difficulty of implementing this in the current QEMU system,
> as it would touch different subsystems like IORT and platform device,
> since the passthrough devices would be attached to different vIOMMUs.
> 
> Yet, given the situations above, it's likely the best by duplicating
> the vIOMMU instance corresponding to the number of the physical SMMU
> instances.
> 
> So, I am sending this email to collect opinions on this and see what
> would be a potential TODO list if we decide to go on this path.
> 
> Thanks
> Nicolin