linux-kernel - Re: [PATCH v11 05/13] iommu: Add attach/detach_dev

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Message-ID: <d538aa77-ac9a-e436-5558-e97e9c68d222@linux.intel.com>
Date:   Tue, 23 Aug 2022 15:30:21 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     baolu.lu@...ux.intel.com, 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>,
        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 v11 05/13] iommu: Add attach/detach_dev_pasid iommu
 interface

On 2022/8/18 21:33, Jason Gunthorpe wrote:
> 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()?

Sure. I will fix this in the next version.

> 
>> +	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.

Updated.

> 
>> +/*
>> + * 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?

It is checked in the function to make sure that the detached domain is
the same one as the previous attached one.

> 
>> +/*
>> + * 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?

"... the returned domain pointer could only be used before 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)
>> +{
>> +	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.

Removed.

> But otherwise this overall thing seems fine to me

Thank you!

Best regards,
baolu