RE: [PATCH v3 08/17] iommufd: IOMMU_HWPT_ALLOC allocation with user data

[Tian, Kevin]

> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Sent: Saturday, July 29, 2023 1:56 AM
> 
> On Mon, Jul 24, 2023 at 04:03:57AM -0700, Yi Liu wrote:
> 
> > +	switch (pt_obj->type) {
> > +	case IOMMUFD_OBJ_IOAS:
> > +		ioas = container_of(pt_obj, struct iommufd_ioas, obj);
> > +		break;
> > +	case IOMMUFD_OBJ_HW_PAGETABLE:
> > +		/* pt_id points HWPT only when hwpt_type
> is !IOMMU_HWPT_TYPE_DEFAULT */
> > +		if (cmd->hwpt_type == IOMMU_HWPT_TYPE_DEFAULT) {
> > +			rc = -EINVAL;
> > +			goto out_put_pt;
> > +		}
> > +
> > +		parent = container_of(pt_obj, struct iommufd_hw_pagetable,
> obj);
> > +		/*
> > +		 * Cannot allocate user-managed hwpt linking to
> auto_created
> > +		 * hwpt. If the parent hwpt is already a user-managed hwpt,
> > +		 * don't allocate another user-managed hwpt linking to it.
> > +		 */
> > +		if (parent->auto_domain || parent->parent) {
> > +			rc = -EINVAL;
> > +			goto out_put_pt;
> > +		}
> > +		ioas = parent->ioas;
> 
> Why do we set ioas here? I would think it should be NULL.
> 
> I think it is looking like a mistake to try and re-use
> iommufd_hw_pagetable_alloc() directly for the nested case. It should
> not have a IOAS argument, it should not do enforce_cc, or iopt_*
> functions

enforce_cc is still required since it's about memory accesses post
page table walking (no matter the walked table is single stage or
nested).

> 
> So must of the function is removed. Probably better to make a new
> ioas-less function for the nesting case.
> 
> > diff --git a/drivers/iommu/iommufd/main.c
> b/drivers/iommu/iommufd/main.c
> > index 510db114fc61..5f4420626421 100644
> > --- a/drivers/iommu/iommufd/main.c
> > +++ b/drivers/iommu/iommufd/main.c
> > @@ -426,7 +426,7 @@ static const struct iommufd_ioctl_op
> iommufd_ioctl_ops[] = {
> >  	IOCTL_OP(IOMMU_GET_HW_INFO, iommufd_get_hw_info, struct
> iommu_hw_info,
> >  		 __reserved),
> >  	IOCTL_OP(IOMMU_HWPT_ALLOC, iommufd_hwpt_alloc, struct
> iommu_hwpt_alloc,
> > -		 __reserved),
> > +		 data_uptr),
> 
> Nono, these can never change once we put them it. It specifies the
> hard minimum size that userspace must provide. If userspace gives less
> than this then the ioctl always fails. Changing it breaks all the
> existing software.
> 
> The core code ensures that the trailing part of the cmd struct is
> zero'd the extended implementation must cope with Zero'd values, which
> this does.
> 

Ideally expanding uAPI structure size should come with new flag bits.

this one might be a bit special. hwpt_alloc() series was just queued to
iommufd-next. If the nesting series could come together in one cycle
then probably changing it in current way is fine since there is no
existing software. Otherwise we need follow common practice. 😊