RE: [PATCH v4 09/17] iommufd: Data structure to provide IOVA to PFN mapping

Tian, Kevin
2022-11-16T00:10:19Z

> From: Jason Gunthorpe <jgg@nvidia.com>
> Sent: Tuesday, November 15, 2022 11:05 PM
>
> On Tue, Nov 15, 2022 at 03:13:56AM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@nvidia.com>
> > > Sent: Tuesday, November 15, 2022 2:44 AM
> > >
> > > On Mon, Nov 14, 2022 at 07:28:47AM +0000, Tian, Kevin wrote:
> > > > > From: Jason Gunthorpe <jgg@nvidia.com>
> > > > > Sent: Tuesday, November 8, 2022 8:49 AM
> > > > >
> > > > > +
> > > > > +/*
> > > > > + * Automatically find a block of IOVA that is not being used and not
> > > reserved.
> > > > > + * Does not return a 0 IOVA even if it is valid.
> > > >
> > > > what is the problem with 0? should this be documented in uAPI?
> > >
> > > 0 is commonly used as an errant value for uninitialized things. We
> > > don't automatically map it into a process mm because it can cause
> > > security problems if we don't trap a bogus 0/NULL pointer reference.
> > >
> > > The same logic applies here too, the allocator should not return 0 to
> > > reserve it as an unmapped IOVA page to catch bugs.
> >
> > CPU doesn't reference IOVA. Where do such bugs exist?
>
> SW is always buggy and SW programs the DMA address, so it could leave
> a 0 behind or something during the programming.

address 0 is never a bug in DMA to IOVA. if it is, it will be out of the
aperture or in the reserved IOVA list.

DMA API is also a auto-iova scheme from driver p.o.v while it doesn't
impose any restriction on address 0.

> > > > > + if (!__alloc_iova_check_used(&allowed_span, length,
> > > > > + iova_alignment,
> page_offset))
> > > > > + continue;
> > > > > +
> > > > > + interval_tree_for_each_span(&area_span, &iopt-
> >area_itree,
> > > > > + allowed_span.start_used,
> > > > > + allowed_span.last_used) {
> > > > > + if (!__alloc_iova_check_hole(&area_span,
> length,
> > > > > + iova_alignment,
> > > > > + page_offset))
> > > > > + continue;
> > > > > +
> > > > > +
> interval_tree_for_each_span(&reserved_span,
> > > > > + &iopt-
> >reserved_itree,
> > > > > +
> area_span.start_used,
> > > > > +
> area_span.last_used) {
> > > > > + if (!__alloc_iova_check_hole(
> > > > > + &reserved_span, length,
> > > > > + iova_alignment,
> page_offset))
> > > > > + continue;
> > > >
> > > > this could be simplified by double span.
> > >
> > > It is subtly not compatible, the double span looks for used areas.
> > > This is looking for a used area in the allowed_itree, a hole in the
> > > area_itree, and a hole in the reserved_itree.
> >
> > the inner two loops can be replaced by double span, since both
> > are skipping used areas.
>
> The 2nd loop is looking for a used on allowed and the 3rd loop is
> looking for a hole in reserved. To fix it we'd have to invert allowed
> to work like reserved - which complicates the uAPI code.

The 1st loop finds an allowed range which can hold requested length

The 2nd loop finds an *unused* hole in the allowed range

The 3rd loop further looks for a hole in reserved.

last two both try to find a hole.

>
> > > > > + if (iopt->disable_large_pages)
> > > > > + new_iova_alignment = PAGE_SIZE;
> > > > > + else
> > > > > + new_iova_alignment = 1;
> > > >
> > > > I didn't understand why we start searching alignment from a
> > > > smaller value when large pages is enabled. what is the
> > > > connection here?
> > >
> > > 'disable_large_pages' is a tiny bit misnamed, what it really does is
> > > ensure that every iommu_map call is exactly PAGE_SIZE, not more (large
> > > pages) and not less (what this is protecting against).
> > >
> > > So if a domain has less than PAGE_SIZE we upgrade to
> > > PAGE_SIZE. Otherwise we allow using the lowest possible alignment.
> > >
> > > This allows userspace to always work in PAGE_SIZE units without fear
> > > of problems, eg with sub-page-size units becoming weird or something.
> >
> > above are good stuff in a comment.
>
> This is the comment:
>
> /*
> * This is part of the VFIO compatibility support for VFIO_TYPE1_IOMMU.
> That
> * mode permits splitting a mapped area up, and then one of the splits is
> * unmapped. Doing this normally would cause us to violate our invariant of
> * pairing map/unmap. Thus, to support old VFIO compatibility disable
> support
> * for batching consecutive PFNs. All PFNs mapped into the iommu are done
> in
> * PAGE_SIZE units, not larger or smaller.
> */
> static int batch_iommu_map_small(struct iommu_domain *domain,
> unsigned long iova, phys_addr_t paddr,
> size_t size, int prot)
>

I meant a comment in iopt_calculate_iova_alignment().