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Date:   Thu, 23 Mar 2023 09:16:51 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Nicolin Chen <nicolinc@nvidia.com>
Cc:     "Tian, Kevin" <kevin.tian@intel.com>,
        Robin Murphy <robin.murphy@arm.com>,
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Subject: Re: [PATCH v1 14/14] iommu/arm-smmu-v3: Add
 arm_smmu_cache_invalidate_user
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On Wed, Mar 22, 2023 at 01:43:59PM -0700, Nicolin Chen wrote:

> > So I think for ARM we want to reflect the physical SMMU instances into
> > vSMMU instances and that feels best done by having a unique S2
> > iommu_domain for each SMMU instance. Then we know that an invalidation
> > for a SMMU instance is delivered to that S2's singular CMDQ and things
> > like vCMDQ become possible.
> 
> In that environment, do we still need a VMID unification?

If each S2 is per-smmu-instance then the VMID can be local to the SMMU
instance

> > > Our approach of setting up a stage-2 mapping in QEMU is to
> > > map the entire guest memory. I don't see a point in having
> > > a separate S2 domain, even if there are multiple instances?
> > 
> > And then this is the drawback, we don't really want to have duplicated
> > S2 page tables in the system for every stage 2.
> > 
> > Maybe we have made a mistake by allowing the S2 to be an unmanaged
> > domain. Perhaps we should create the S2 out of an unmanaged domain
> > like the S1.
> > 
> > Then the rules could be
> >  - Unmanaged domain can be used with every smmu instance, only one
> >    copy of the page table. The ASID in the iommu_domain is
> >    kernel-global
> >  - S2 domain is a child of a shared unmanaged domain. It can be used
> >    only with the SMMU it is associated with, it has a per-SMMU VM ID
> >  - S1 domain is a child of a S2 domain, it can be used only with the
> >    SMMU it's S2 is associated with, just because
> 
> The actual S2 pagetable has to stay at the unmanaged domain
> for IOAS_MAP, while we maintain an s2_cfg data structure in
> the shadow S2 domain per SMMU instance that has its own VMID
> but a shared S2 page table pointer?

Yes

> Hmm... Feels very complicated to me. How does that help?

It de-duplicates the page table across multiple SMMU instances.

> > So, I have been exploring a different approach by creating an
> > > internal multiplication inside VCMDQ...
> > 
> > How can that work?
> > 
> > You'd have to have the guest VM to know to replicate to different
> > vCMDQ's? Which isn't the standard SMMU programming model anymore..
> 
> VCMDQ has multiple VINTFs (Virtual Interfaces) that's supposed
> to be used by the host to expose to multiple VMs.
> 
> In a multi-SMMU environment, every single SMMU+VCMDQ instance
> would have one VINTF only that contains one or more VCMDQs. In
> this case, passthrough devices behind different physical SMMU
> instances are straightforwardly attached to different vSMMUs.

Yes, this is the obvious simple impementation

> However, if we can't have multiple vSMMU instances, the guest
> VM (its HW) would enable multiple VINTFs corresponding to the
> number of physical SMMU/VCMDQ instances, for devices to attach
> accordingly. That means I need to figure out a way to pin the
> devices onto those VINTFs, by somehow passing their physical
> SMMU IDs. 

And a way to request the correctly bound vCMDQ from the guest as well.
Sounds really messsy, I'd think multi-smmu is the much cleaner choice

Jason