From mboxrd@z Thu Jan  1 00:00:00 1970
From: Alexander Graf <agraf@suse.de>
Subject: VFIO iommu page size masking
Date: Fri, 13 Feb 2015 03:41:59 +0100
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Cc: KVM <kvm@vger.kernel.org>, Varun Sethi <Varun.Sethi@freescale.com>,
	Will Deacon <will.deacon@arm.com>
To: Alex Williamson <alex.williamson@redhat.com>
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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