linux-kernel - Re: [PATCH v12 05/17] iommu: Add attach/detach_dev

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Message-ID: <YwjauaF1sTP6l2j9@nvidia.com>
Date:   Fri, 26 Aug 2022 11:37:45 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Vinod Koul <vkoul@...nel.org>,
        Eric Auger <eric.auger@...hat.com>,
        Liu Yi L <yi.l.liu@...el.com>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Zhangfei Gao <zhangfei.gao@...aro.org>,
        Zhu Tony <tony.zhu@...el.com>, iommu@...ts.linux.dev,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jean-Philippe Brucker <jean-philippe@...aro.org>
Subject: Re: [PATCH v12 05/17] iommu: Add attach/detach_dev_pasid iommu
 interface

On Fri, Aug 26, 2022 at 08:11:29PM +0800, Lu Baolu wrote:

> + * iommu_get_domain_for_dev_pasid() - Retrieve domain for @pasid of @dev
> + * @dev: the queried device
> + * @pasid: the pasid of the device
> + * @type: matched domain type, 0 for any match
> + *
> + * This is a variant of iommu_get_domain_for_dev(). It returns the existing
> + * domain attached to pasid of a device. It's only for internal use of the
> + * IOMMU subsystem. 

If it is only for external use then why is it exported?

I would add something like:

 Callers must hold a lock around this function, and both
 iommu_attach/detach_dev_pasid() whenever a domain of type is being
 manipulated. This API does not internally resolve races with
 attach/detach.

> + * detaching from the device PASID.
> + *
> + * Return: attached domain on success, NULL otherwise.
> + */
> +struct iommu_domain *iommu_get_domain_for_dev_pasid(struct device *dev,
> +						    ioasid_t pasid,
> +						    unsigned int type)
> +{
> +	struct iommu_domain *domain;
> +	struct iommu_group *group;
> +
> +	group = iommu_group_get(dev);
> +	if (!group)
> +		return NULL;
> +	/*
> +	 * The xarray protects its internal state with RCU. Hence the domain
> +	 * obtained is either NULL or fully formed.
> +	 */

This has nothing to do with RCU

xa_lock() is used to ensure that the domain pointer remains valid
while we check the type since it blocks concurrent xa_erase().

Jason