Re: [PATCH v2 5/7] iommu/riscv: Device directory management.

Jason Gunthorpe
2024-04-24T23:07:18Z

On Wed, Apr 24, 2024 at 04:01:04PM -0700, Tomasz Jeznach wrote:
> > > + /* Update existing or allocate new entries in device directory */
> > > + for (i = 0; i < fwspec->num_ids; i++) {
> > > + dc = riscv_iommu_get_dc(iommu, fwspec->ids[i], !iommu_domain);
> > > + if (!dc && !iommu_domain)
> > > + continue;
> > > + if (!dc)
> > > + return -ENODEV;
> >
> > But if this fails some of the fwspecs were left in a weird state ?
> >
> > Drivers should try hard to have attach functions that fail and make no
> > change at all or fully succeed.
> >
> > Meaning ideally preallocate any required memory before doing any
> > change to the HW visable structures.
>
> Good point. Done.
> Looking at the fwspec->ids[] I'm assuming nobody will add/modify the
> IDs after iommu_probe_device() completes.

Yes

> > > + /* Swap device context, update TC valid bit as the last operation */
> > > + xchg64(&dc->fsc, fsc);
> > > + xchg64(&dc->ta, ta);
> > > + xchg64(&dc->tc, tc);
> >
> > This doesn't loook right? When you get to adding PAGING suport fsc has
> > the page table pfn and ta has the cache tag, so this will end up
> > tearing the data for sure, eg when asked to replace a PAGING domain
> > with another PAGING domain? That will create a functional/security
> > problem, right?
> >
> > I would encourage you to re-use the ARM sequencing code, ideally moved
> > to some generic helper library. Every iommu driver dealing with
> > multi-quanta descriptors seems to have this same fundamental
> > sequencing problem.
> >
>
> Good point. Reworked.

I suppose by force clearing the v bit before starting the sequence?

That is OK but won't support some non-embedded focused features in the
long run. It is a good approach to get the driver landed though.

> > The release_domain has landed too now. Please don't invent weird NULL
> > domain types that have special meaning. I assume clearing the V bit is
> > a blocking behavior? So please implement a proper blocking domain and
> > set release_domain = &riscv_iommu_blocking and just omit this release
> > function.
> >
>
> Updated to use release_domain, should be cleaner now.
> Clearing TC.V is a blocking (but noisy) behavior, should be fine for
> release domain where devices should be quiesced already.

blocking is fine to be noisy.

Jason