linux-kernel - Re: [PATCH v3 3/5] iommu: Add iommu_get_domain_for_dev

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Message-ID: <20250818143949.GO802098@nvidia.com>
Date: Mon, 18 Aug 2025 11:39:49 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: robin.murphy@....com, joro@...tes.org, bhelgaas@...gle.com,
	will@...nel.org, robin.clark@....qualcomm.com, yong.wu@...iatek.com,
	matthias.bgg@...il.com, angelogioacchino.delregno@...labora.com,
	thierry.reding@...il.com, vdumpa@...dia.com, jonathanh@...dia.com,
	rafael@...nel.org, lenb@...nel.org, kevin.tian@...el.com,
	yi.l.liu@...el.com, baolu.lu@...ux.intel.com,
	linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	linux-mediatek@...ts.infradead.org, linux-tegra@...r.kernel.org,
	linux-acpi@...r.kernel.org, linux-pci@...r.kernel.org,
	patches@...ts.linux.dev, pjaroszynski@...dia.com, vsethi@...dia.com,
	helgaas@...nel.org, etzhao1900@...il.com
Subject: Re: [PATCH v3 3/5] iommu: Add iommu_get_domain_for_dev_locked()
 helper

On Mon, Aug 11, 2025 at 03:59:10PM -0700, Nicolin Chen wrote:

> Introduce a new iommu_get_domain_for_dev_locked() helper to be used by
> those drivers in a context that is already under the protection of the
> group->mutex, e.g. those attach_dev callback functions. And roll out the
> new helper to all the existing IOMMU drivers.

I still don't much care for this, I think it would be better to
approach this problem by passing the old domain to the driver callback:

> @@ -390,7 +390,7 @@ static int qcom_iommu_attach_dev(struct iommu_domain *domain, struct device *dev
>  static int qcom_iommu_identity_attach(struct iommu_domain *identity_domain,
>  				      struct device *dev)
>  {
> -	struct iommu_domain *domain = iommu_get_domain_for_dev(dev);
> +	struct iommu_domain *domain = iommu_get_domain_for_dev_locked(dev);

Because this is a very common pattern in drivers.

Once that is done we can see what calls to iommu_get_domain_for_dev()
are even left, arguably we should be trying to eliminate this badly
locked thing...

Jason