Re: [PATCH v4 07/17] iommufd: Add user-managed hw_pagetable support

[Nicolin Chen]

On Tue, Sep 26, 2023 at 01:14:47AM -0700, Tian, Kevin wrote:
> > From: Liu, Yi L <yi.l.liu@xxxxxxxxx>
> > Sent: Thursday, September 21, 2023 3:51 PM
> >
> > +static void iommufd_user_managed_hwpt_abort(struct iommufd_object
> > *obj)
> > +{
> > +     struct iommufd_hw_pagetable *hwpt =
> > +             container_of(obj, struct iommufd_hw_pagetable, obj);
> > +
> > +     /* The parent->mutex must be held until finalize is called. */
> > +     lockdep_assert_held(&hwpt->parent->mutex);
> > +
> > +     iommufd_hw_pagetable_destroy(obj);
> > +}
> 
> Can you elaborate what exactly is protected by parent->mutex?
> 
> My gut-feeling that the child just grabs a refcnt on the parent
> object. It doesn't change any other data of the parent.

Ah, you are right. It's added here just for symmetry so we wouldn't
end up with something like:
	if (!hwpt->user_managed)
		mutex_lock(&hwpt->mutex);
	alloc_fn();
	if (!hwpt->user_managed)
		mutex_unlock(&hwpt->mutex);

Perhaps I should move the pair of mutex calls to the kernel-managed
hwpt allocator.

> > +/**
> > + * iommufd_user_managed_hwpt_alloc() - Get a user-managed
> > hw_pagetable
> > + * @ictx: iommufd context
> > + * @pt_obj: Parent object to an HWPT to associate the domain with
> > + * @idev: Device to get an iommu_domain for
> > + * @flags: Flags from userspace
> > + * @hwpt_type: Requested type of hw_pagetable
> > + * @user_data: user_data pointer
> > + * @dummy: never used
> > + *
> > + * Allocate a new iommu_domain (must be IOMMU_DOMAIN_NESTED) and
> > return it as
> > + * a user-managed hw_pagetable.
> 
> Add some text to highlight the requirement being a parent, e.g. not
> an auto domain, must be capable of being a parent, etc.

OK.

> > +     case IOMMUFD_OBJ_HW_PAGETABLE:
> > +             parent = container_of(pt_obj, struct iommufd_hw_pagetable,
> > obj);
> > +             /* No user-managed HWPT on top of an user-managed one
> > */
> > +             if (parent->user_managed) {
> > +                     rc = -EINVAL;
> > +                     goto out_put_pt;
> > +             }
> 
> move to alloc_fn().

Though being a bit covert, this is actually to avoid a data buffer
allocation in the common pathway before calling alloc_fn(), which
is added in the following patch. And the reason why it's in the
common function is because we previously support a kernel-managed
hwpt allocation with data too.

But now, I think we can just move this sanity and data allocation
together into the user-managed hwpt allocator.

Thanks
Nicolin