[PATCH 2/4] iommu: add ARM LPAE page table allocator

Will Deacon will.deacon at arm.com
Tue Dec 2 01:41:56 PST 2014

On Mon, Dec 01, 2014 at 08:21:58PM +0000, Laurent Pinchart wrote:
> On Monday 01 December 2014 17:23:15 Will Deacon wrote:
> > On Sun, Nov 30, 2014 at 11:29:46PM +0000, Laurent Pinchart wrote:
> > > On Thursday 27 November 2014 11:51:16 Will Deacon wrote:
> > > > +     /* Looking good; allocate a pgd */
> > > > +     data->pgd = alloc_pages_exact(1UL << data->pg_shift,
> > > > +                                   GFP_KERNEL | __GFP_ZERO);
> > > 
> > > data->pg_shift is computed as __ffs(cfg->pgsize_bitmap). 1UL <<
> > > data->pg_shift will thus be equal to the smallest page size supported by
> > > the IOMMU. This will thus allocate 4kB, 16kB or 64kB depending on the
> > > IOMMU configuration. However, if I'm not mistaken the top-level directory
> > > needs to store one entry per largest supported page size. That's 4, 128
> > > or 8 entries depending on the configuration. You're thus over-allocating.
> > 
> > Yeah, I'll take a closer look at this. The size of the pgd really depends
> > on the TxSZ configuration, which in turn depends on the ias and the page
> > size. There are also alignment constraints to bear in mind, but I'll see
> > what I can do (as it stands, over-allocating will always work).
> Beside wasting memory, the code also doesn't reflect the requirements. It 
> works by chance, meaning it could break later.

It won't break, as the maximum size *is* bounded by a page for stage-1
and we already handle stage-2 concatenation correctly.

> That's why I'd like to see this 
> being fixed. Can't the size be computed with something like
> 	size = (1 << (ias - data->levels * data->pg_shift))
> 	     * sizeof(arm_lpae_iopte);
> (please add a proper detailed comment to explain the computation, as the 
> meaning is not straightforward)

That's actually the easy part. The harder part is getting the correct
alignment, which means managing by own kmem_cache on a per-ops basis. That
feels like overkill to me and we also need to make sure that we don't screw
up the case of concatenated pgds at stage-2. On top of that, since each
cache would be per-ops, I'm not even sure we'd save anything (the slab
allocators all operate on pages afaict).

If I use alloc_page_exact, we'll still have some wasteage, but it would 
be less for the case where the CPU page size is smaller than the SMMU page
size. Do you think that's worth the extra complexity? We allocate full pages
at all levels after the pgd, so the wasteage is relatively small.

An alternative would be preinitialising some caches for `likely' pgd sizes,
but that's also horrible, especially if the kernel decides that it doesn't
need a bunch of the configurations at runtime.


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