Re: [PATCH v2 14/18] iommu/arm-smmu-v3: Support domains with shared CDs

[Baolu Lu]

On 6/8/23 9:39 PM, Jason Gunthorpe wrote:
> On Thu, Jun 08, 2023 at 10:39:23AM +0800, Baolu Lu wrote:
> > On 6/7/23 7:59 PM, Jason Gunthorpe wrote:
> > > On Wed, Jun 07, 2023 at 12:06:07AM +0530, Michael Shavit wrote:
> > > > > What we definately shouldn't do is try to have different SVA
> > > > > iommu_domain's pointing at the same ASID. That is again making SVA
> > > > > special, which we are trying to get away from 😄
> > > > Fwiw, this change is preserving the status-quo in that regard;
> > > > arm-smmu-v3-sva.c is already doing this. But yes, I agree that
> > > > resolving the limitation is a better long term solution... and
> > > > something I can try to look at further.
> > > I suppose we also don't really have a entirely clear picture what
> > > allocating multiple SVA domains should even do in the iommu driver.
> > >
> > > The driver would like to share the ASID, but things are much cleaner
> > > for everything if the driver model has ASID 1:1 with the iommu_domain.
> > This means that each ASID should be mapped to a single IOMMU domain.
> > This is conceptually right as iommu_domain represents a hardware page
> > table. For SVA, it's an mm_struct.
> >
> > So in my mind, each sva_domain should have a 1:1 relationship with an
> > mm_struct.
> If we want to support multiple iommu drivers then we should support
> multiple iommu_domains per mm_struct so that each driver can have its
> own. In this world if each instance wants its own iommu_domain it is
> not a big deal.
>
> Drivers that can share iommu_domains across instances should probably
> also share sva iommu_domains across instances.
>
> Most real systems have only one iommu driver and we'd like the good
> iommu drivers to be able to share domains across instances, so we'd
> expect only 1 iommu_domain per mm struct.

Yes. You are right. I overlooked the multiple-drivers case. So we stay
on the same page now.

Best regards,
baolu