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

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Date: Wed, 24 Apr 2024 20:07:15 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Tomasz Jeznach <tjeznach@...osinc.com>
Cc: Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Robin Murphy <robin.murphy@....com>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Anup Patel <apatel@...tanamicro.com>,
	Sunil V L <sunilvl@...tanamicro.com>,
	Nick Kossifidis <mick@....forth.gr>,
	Sebastien Boeuf <seb@...osinc.com>,
	Rob Herring <robh+dt@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, devicetree@...r.kernel.org,
	iommu@...ts.linux.dev, linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux@...osinc.com
Subject: Re: [PATCH v2 5/7] iommu/riscv: Device directory management.

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