Re: [PATCH v1 2/6] arm/smmu: Add auxiliary domain support for arm-smmuv2

[Jordan Crouse]

On Wed, Mar 18, 2020 at 04:43:07PM -0700, Rob Clark wrote:
> On Wed, Mar 18, 2020 at 3:48 PM Will Deacon <will@xxxxxxxxxx> wrote:
> >
> > On Tue, Jan 28, 2020 at 03:16:06PM -0700, Jordan Crouse wrote:
> > > Support auxiliary domains for arm-smmu-v2 to initialize and support
> > > multiple pagetables for a single SMMU context bank. Since the smmu-v2
> > > hardware doesn't have any built in support for switching the pagetable
> > > base it is left as an exercise to the caller to actually use the pagetable.
> > >
> > > Aux domains are supported if split pagetable (TTBR1) support has been
> > > enabled on the master domain.  Each auxiliary domain will reuse the
> > > configuration of the master domain. By default the a domain with TTBR1
> > > support will have the TTBR0 region disabled so the first attached aux
> > > domain will enable the TTBR0 region in the hardware and conversely the
> > > last domain to be detached will disable TTBR0 translations.  All subsequent
> > > auxiliary domains create a pagetable but not touch the hardware.
> > >
> > > The leaf driver will be able to query the physical address of the
> > > pagetable with the DOMAIN_ATTR_PTBASE attribute so that it can use the
> > > address with whatever means it has to switch the pagetable base.
> > >
> > > Following is a pseudo code example of how a domain can be created
> > >
> > >  /* Check to see if aux domains are supported */
> > >  if (iommu_dev_has_feature(dev, IOMMU_DEV_FEAT_AUX)) {
> > >        iommu = iommu_domain_alloc(...);
> > >
> > >        if (iommu_aux_attach_device(domain, dev))
> > >                return FAIL;
> > >
> > >       /* Save the base address of the pagetable for use by the driver
> > >       iommu_domain_get_attr(domain, DOMAIN_ATTR_PTBASE, &ptbase);
> > >  }
> >
> > I'm not really understanding what the pagetable base gets used for here and,
> > to be honest with you, the whole thing feels like a huge layering violation
> > with the way things are structured today. Why doesn't the caller just
> > interface with io-pgtable directly?
> >
> > Finally, if we need to support context-switching TTBR0 for a live domain
> > then that code really needs to live inside the SMMU driver because the
> > ASID and TLB management necessary to do that safely doesn't belong anywhere
> > else.
> 
> Hi Will,
> 
> We do in fact need live domain switching, that is really the whole
> point.  The GPU CP (command processor/parser) is directly updating
> TTBR0 and triggering TLB flush, asynchronously from the CPU.

Right. This is entirely done in hardware with a GPU that has complete access to
the context bank registers. All the driver does is send the PTBASE to the
command stream see [1] and especially [2] (look for CP_SMMU_TABLE_UPDATE).

As for interacting with the io-pgtable directly I would love to do that but it
would need some new infrastructure to either pull the io-pgtable from the aux
domain or to create an io-pgtable ourselves and pass it for use by the aux
domain. I'm not sure if that is better for the layering violation.

> And I think the answer about ASID is easy (on current hw).. it must be zero[*].

Right now the GPU microcode still uses TLBIALL. I want to assign each new aux
domain its own ASID in the hopes that we could some day change that but for now
having a uinque ASID doesn't help.

Jordan

[1] https://patchwork.freedesktop.org/patch/351089/
[2] https://patchwork.freedesktop.org/patch/351090/

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