Re: Multiple vIOMMU instance support in QEMU?

[Eric Auger <eric.auger@redhat.com>]

Hi Nicolin,

On 5/18/23 05:22, Nicolin Chen wrote:
> 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.
Well if there is a real production use case behind the requirement of
having mutliple vSMMUs (and more generally vIOMMUs) sure you can go
ahead. I just wanted to warn you that as far as I know multiple vIOMMUS
are not supported even on Intel iommu and virtio-iommu. Let's add Peter
Xu in CC. I foresee added complexicity with regard to how you define the
RID scope of each vIOMMU, ACPI table generation, impact on arm-virt
machine options, how you pass the feature associated to each instance,
notifier propagation impact? And I don't evoke the VCMDQ feat addition.
We are still far from having a singleton QEMU nested stage SMMU
implementation at the moment but I understand you may want to feed the
pipeline to pave the way for enhanced use cases.

Thanks

Eric
>>
>> 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