Re: [PATCH v2 7/7] iommu/riscv: Paging domain support

From: Jason Gunthorpe <jgg@ziepe.ca>
To: Baolu Lu <baolu.lu@linux.intel.com>
Cc: Tomasz Jeznach <tjeznach@rivosinc.com>,
	Joerg Roedel <joro@8bytes.org>, Will Deacon <will@kernel.org>,
	Robin Murphy <robin.murphy@arm.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Anup Patel <apatel@ventanamicro.com>,
	Sunil V L <sunilvl@ventanamicro.com>,
	Nick Kossifidis <mick@ics.forth.gr>,
	Sebastien Boeuf <seb@rivosinc.com>,
	Rob Herring <robh+dt@kernel.org>,
	Krzysztof Kozlowski <krzk+dt@kernel.org>,
	Conor Dooley <conor+dt@kernel.org>,
	devicetree@vger.kernel.org, iommu@lists.linux.dev,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	linux@rivosinc.com
Subject: Re: [PATCH v2 7/7] iommu/riscv: Paging domain support
Date: Mon, 22 Apr 2024 16:30:59 -0300	[thread overview]
Message-ID: <20240422193059.GC231144@ziepe.ca> (raw)
In-Reply-To: <2f2750cd-a5bf-4486-8f50-c93d246f8b0c@linux.intel.com>

On Mon, Apr 22, 2024 at 01:21:05PM +0800, Baolu Lu wrote:
> > +	/* Track domain to devices mapping. */
> > +	if (bond)
> > +		list_add_rcu(&bond->list, &domain->bonds);
> > +
> > +	/* Remove tracking from previous domain, if needed. */
> > +	iommu_domain = iommu_get_domain_for_dev(dev);
> 
> Calling iommu_get_domain_for_dev() in the domain attaching path is very
> fragile because it heavily depends on the order of calling the attach
> callback and setting the domain pointer in the core.

We have a couple places doing this already, the core code accomodates
it well enough for deleting from a list.. So I think it is OK to keep
doing.

> Perhaps the driver can use dev_iommu_priv_set/get() to keep the active
> domain in the per-device private data?
> 
> > +	if (iommu_domain && !!(iommu_domain->type & __IOMMU_DOMAIN_PAGING)) {
> > +		domain = iommu_domain_to_riscv(iommu_domain);
> > +		bond = NULL;
> > +		rcu_read_lock();
> > +		list_for_each_entry_rcu(b, &domain->bonds, list) {
> > +			if (b->dev == dev) {
> > +				bond = b;
> > +				break;
> > +			}
> > +		}
> > +		rcu_read_unlock();

But now that I look again, this is not safe, you have to hold some
kind of per-domain lock to mutate the list. rcu_*read*_lock() cannot
be used for write.

Jason