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

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Message-ID: <20240422193059.GC231144@ziepe.ca>
Date: Mon, 22 Apr 2024 16:30:59 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Baolu Lu <baolu.lu@...ux.intel.com>
Cc: Tomasz Jeznach <tjeznach@...osinc.com>, 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 7/7] iommu/riscv: Paging domain support

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