Re: [PATCH v2 17/19] iommu/arm-smmu-v3: Add arm_smmu_viommu_cache_invalidate

[Baolu Lu]

On 2024/9/11 15:20, Nicolin Chen wrote:
> On Wed, Sep 11, 2024 at 06:25:16AM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe<jgg@xxxxxxxxxx>
> > > Sent: Friday, September 6, 2024 2:22 AM
> > >
> > > On Thu, Sep 05, 2024 at 11:00:49AM -0700, Nicolin Chen wrote:
> > > > On Thu, Sep 05, 2024 at 01:20:39PM -0300, Jason Gunthorpe wrote:
> > > > > On Tue, Aug 27, 2024 at 09:59:54AM -0700, Nicolin Chen wrote:
> > > > >
> > > > > > +static int arm_smmu_viommu_cache_invalidate(struct
> > > iommufd_viommu *viommu,
> > > > > > +                                           struct iommu_user_data_array
> > > *array)
> > > > > > +{
> > > > > > +       struct iommu_domain *domain =
> > > iommufd_viommu_to_parent_domain(viommu);
> > > > > > +
> > > > > > +       return __arm_smmu_cache_invalidate_user(
> > > > > > +                       to_smmu_domain(domain), viommu, array);
> > > > > I'd like to have the viommu struct directly hold the VMID. The nested
> > > > > parent should be sharable between multiple viommus, it doesn't make
> > > > > any sense that it would hold the vmid.
> > > > >
> > > > > This is struggling because it is trying too hard to not have the
> > > > > driver allocate the viommu, and I think we should just go ahead and do
> > > > > that. Store the vmid, today copied from the nesting parent in the vmid
> > > > > private struct. No need for iommufd_viommu_to_parent_domain(), just
> > > > > rework the APIs to pass the vmid down not a domain.
> > > > OK. When I designed all this stuff, we still haven't made mind
> > > > about sharing the s2 domain, i.e. moving the VMID, which might
> > > > need a couple of more patches to achieve.
> > > Yes, many more patches, and don't try to do it now.. But we can copy
> > > the vmid from the s2 and place it in the viommu struct during
> > > allocation time.
> > does it assume that a viommu object cannot span multiple physical
> > IOMMUs so there is only one vmid per viommu?
> I think so. One the reasons of introducing vIOMMU is to maintain
> the shareability across physical IOMMUs at the s2 HWPT_PAGING.

My understanding of VMID is something like domain id in x86 arch's. Is
my understanding correct?

If a VMID for an S2 hwpt is valid on physical IOMMU A but has already
been allocated for another purpose on physical IOMMU B, how can it be
shared across both IOMMUs? Or the VMID is allocated globally?

Thanks,
baolu