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

[Baolu Lu]

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