VFIO iommu page size masking

[Alexander Graf <agraf@suse.de>]

Hi Alex,

While trying to get VFIO-PCI working on AArch64 (with 64k page size), I
stumbled over the following piece of code:

> static unsigned long vfio_pgsize_bitmap(struct vfio_iommu *iommu)
> {
>         struct vfio_domain *domain;
>         unsigned long bitmap = PAGE_MASK;
> 
>         mutex_lock(&iommu->lock);
>         list_for_each_entry(domain, &iommu->domain_list, next)
>                 bitmap &= domain->domain->ops->pgsize_bitmap;
>         mutex_unlock(&iommu->lock);
> 
>         return bitmap;
> }

The SMMU page mask is

[    3.054302] arm-smmu e0a00000.smmu: 	Supported page sizes: 0x40201000

but after this function, we end up supporting one 2MB pages and above.
The reason for that is simple: You restrict the bitmap to PAGE_MASK and
above.

Now the big question is why you're doing that. I don't see why it would
be a problem if the IOMMU maps a page in smaller chunks.

So I tried to patch the code above with s/PAGE_MASK/1UL/ and everything
seems to run fine. But maybe we're not lacking some sanity checks?


Alex