RE: [PATCH RFCv1 08/14] iommufd: Add IOMMU_VIOMMU_SET_DEV_ID ioctl

Tian, Kevin
2024-05-27T01:08:46Z

> From: Jason Gunthorpe <jgg@nvidia.com>
> Sent: Friday, May 24, 2024 9:19 PM
>
> On Fri, May 24, 2024 at 07:13:23AM +0000, Tian, Kevin wrote:
> > I'm curious to learn the real reason of that design. Is it because you
> > want to do certain load-balance between viommu's or due to other
> > reasons in the kernel smmuv3 driver which e.g. cannot support a
> > viommu spanning multiple pSMMU?
>
> Yeah, there is no concept of support for a SMMUv3 instance where it's
> command Q's can only work on a subset of devices.
>
> My expectation was that VIOMMU would be 1:1 with physical iommu
> instances, I think AMD needs this too??
>

Yes this part is clear now regarding to VCMDQ.

But Nicoline said:

"
One step back, even without VCMDQ feature, a multi-pSMMU setup
will have multiple viommus (with our latest design) being added
to a viommu list of a single vSMMU's. Yet, vSMMU in this case
always traps regular SMMU CMDQ, so it can do viommu selection
or even broadcast (if it has to).
"

I don't think there is an arch limitation mandating that?