Re: [PATCH v11 05/13] iommu: Add attach/detach_dev_pasid iommu interface

From: Jason Gunthorpe <jgg@nvidia.com>
To: Lu Baolu <baolu.lu@linux.intel.com>
Cc: Joerg Roedel <joro@8bytes.org>,
	Christoph Hellwig <hch@infradead.org>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Kevin Tian <kevin.tian@intel.com>,
	Ashok Raj <ashok.raj@intel.com>, Will Deacon <will@kernel.org>,
	Robin Murphy <robin.murphy@arm.com>,
	Jean-Philippe Brucker <jean-philippe@linaro.com>,
	Dave Jiang <dave.jiang@intel.com>, Vinod Koul <vkoul@kernel.org>,
	Eric Auger <eric.auger@redhat.com>, Liu Yi L <yi.l.liu@intel.com>,
	Jacob jun Pan <jacob.jun.pan@intel.com>,
	Zhangfei Gao <zhangfei.gao@linaro.org>,
	Zhu Tony <tony.zhu@intel.com>,
	iommu@lists.linux.dev, linux-pci@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Jean-Philippe Brucker <jean-philippe@linaro.org>
Subject: Re: [PATCH v11 05/13] iommu: Add attach/detach_dev_pasid iommu interface
Date: Thu, 18 Aug 2022 10:33:39 -0300	[thread overview]
Message-ID: <Yv4/s6lX6Nq+40tu@nvidia.com> (raw)
In-Reply-To: <20220817012024.3251276-6-baolu.lu@linux.intel.com>

On Wed, Aug 17, 2022 at 09:20:16AM +0800, Lu Baolu wrote:

> +static int __iommu_set_group_pasid(struct iommu_domain *domain,
> +				   struct iommu_group *group, ioasid_t pasid)
> +{
> +	struct iommu_domain *ops_domain;
> +	struct group_device *device;
> +	int ret = 0;
> +
> +	if (domain == group->blocking_domain)
> +		ops_domain = xa_load(&group->pasid_array, pasid);
> +	else
> +		ops_domain = domain;

This seems weird, why isn't this just always

domain->ops->set_dev_pasid()?

> +	if (curr) {
> +		ret = xa_err(curr) ? : -EBUSY;
> +		goto out_unlock;
> +	}
> +
> +	ret = __iommu_set_group_pasid(domain, group, pasid);
> +	if (ret) {
> +		__iommu_set_group_pasid(group->blocking_domain, group, pasid);
> +		xa_erase(&group->pasid_array, pasid);

I was looking at this trying to figure out why we are having
attach/detach semantics vs set and this error handling seems to be the
reason

Lets add a comment because it is subtle thing:

  Setting a PASID to a blocking domain cannot fail, so we can always
  safely error unwind a failure to attach a domain back to the original
  group configuration of the PASID being unused.

> +/*
> + * iommu_detach_device_pasid() - Detach the domain from pasid of device
> + * @domain: the iommu domain.
> + * @dev: the attached device.
> + * @pasid: the pasid of the device.
> + *
> + * The @domain must have been attached to @pasid of the @dev with
> + * iommu_attach_device_pasid().
> + */
> +void iommu_detach_device_pasid(struct iommu_domain *domain, struct device *dev,
> +			       ioasid_t pasid)

Don't pass domain here?

> +/*
> + * iommu_get_domain_for_dev_pasid() - Retrieve domain for @pasid of @dev
> + * @dev: the queried device
> + * @pasid: the pasid of the device
> + *
> + * 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. The caller must take care to avoid any possible
> + * use-after-free case.

How exactly does the caller manage that?

> + *
> + * Return: attached domain on success, NULL otherwise.
> + */
> +struct iommu_domain *
> +iommu_get_domain_for_dev_pasid(struct device *dev, ioasid_t pasid)
> +{
> +	struct iommu_domain *domain;
> +	struct iommu_group *group;
> +
> +	if (!pasid_valid(pasid))
> +		return NULL;

Why bother? If the pasid is not valid then it definitely won't be in the xarray.

But otherwise this overall thing seems fine to me

Jason