Re: [PATCH v6 07/10] iommufd: Add a nested HW pagetable object

[Nicolin Chen <nicolinc@nvidia.com>]

On Tue, Oct 24, 2023 at 03:00:49PM -0300, Jason Gunthorpe wrote:
> On Tue, Oct 24, 2023 at 10:50:58AM -0700, Nicolin Chen wrote:
> 
> > > The point is for the user_data to always be available, the driver
> > > needs to check it if it is passed.
> > > 
> > > This should all be plumbed to allow drivers to also customize their
> > > paging domains too.
> > 
> > We don't have a use case of customizing the paging domains.
> > And our selftest isn't covering this path. Nor the case is
> > supported by the uAPI:
> 
> But this is the design, it is why everything is setup like this - we
> didn't create a new op to allocate nesting domains, we made a flexible
> user allocator.
> 
> > 458- * A kernel-managed HWPT will be created with the mappings from the given
> > 459- * IOAS via the @pt_id. The @data_type for this allocation must be set to
> > 460: * IOMMU_HWPT_DATA_NONE. The HWPT can be allocated as a parent HWPT for a
> > 461- * nesting configuration by passing IOMMU_HWPT_ALLOC_NEST_PARENT via @flags.
> > 462- *
> 
> Yes, that is the reality today. If someone comes to use the more
> complete interface they need to fix that comment..

Ack.

> > Also, if we do passing in the data, we'd need to...
>  
> > 280-static struct iommu_domain *
> > 281-mock_domain_alloc_user(struct device *dev, u32 flags,
> > 282-		       struct iommu_domain *parent,
> > 283:		       const struct iommu_user_data *user_data)
> > 284-{
> > 285-	struct mock_iommu_domain *mock_parent;
> > 286-	struct iommu_hwpt_selftest user_cfg;
> > 287-	int rc;
> > 288-
> > 289:	if (!user_data) {	/* must be mock_domain */
> >
> > ...change this to if (!parent)...
> 
> Yes, this logic is not ideal. The parent is the request for nesting,
> not user_data. user_data is the generic creation parameters, which are
> not supported outside nesting
>  
> Like this:
> 
> --- a/drivers/iommu/iommufd/selftest.c
> +++ b/drivers/iommu/iommufd/selftest.c
> @@ -286,14 +286,12 @@ mock_domain_alloc_user(struct device *dev, u32 flags,
>         int rc;
>  
>         /* must be mock_domain */
> -       if (!user_data) {
> +       if (!parent) {
>                 struct mock_dev *mdev = container_of(dev, struct mock_dev, dev);
>                 bool has_dirty_flag = flags & IOMMU_HWPT_ALLOC_DIRTY_TRACKING;
>                 bool no_dirty_ops = mdev->flags & MOCK_FLAGS_DEVICE_NO_DIRTY;
>  
> -               if (parent)
> -                       return ERR_PTR(-EINVAL);
> -               if (has_dirty_flag && no_dirty_ops)
> +               if (user_data || (has_dirty_flag && no_dirty_ops))
>                         return ERR_PTR(-EOPNOTSUPP);
>                 return __mock_domain_alloc_paging(IOMMU_DOMAIN_UNMANAGED,
>                                                   has_dirty_flag);

Yea.. Then the vt-d driver needs a similar change too (@Yi) as I
found it almost doing the same:
https://lore.kernel.org/linux-iommu/20231024151412.50046-8-yi.l.liu@intel.com/

Thanks
Nic