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

[Baolu Lu <baolu.lu@linux.intel.com>]

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