VFIO iommu page size masking

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;
}

Hi Alex,

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

Quoted 12 lines

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?