On 29/10/15 18:28, Will Deacon wrote:
On Tue, Oct 27, 2015 at 10:00:11AM -0600, Alex Williamson wrote:
On Tue, 2015-10-27 at 15:40 +0000, Will Deacon wrote:
On Fri, Oct 16, 2015 at 09:51:22AM -0600, Alex Williamson wrote:
Would it be possible to add iommu_domain_geometry support to arm-smmu.c?
In addition to this test to verify that DMA cannot bypass the IOMMU, I'd
eventually like to pass the aperture information out through the VFIO
API.  Thanks,

The slight snag here is that we don't know which SMMU in the system the
domain is attached to at the point when the geometry is queried, so I
can't give you an upper bound on the aperture. For example, if there is
an SMMU with a 32-bit input address and another with a 48-bit input

We could play the same horrible games that we do with the pgsize bitmap,
and truncate some global aperture everytime we probe an SMMU device, but
I'd really like to have fewer hacks like that if possible. The root of
the problem is still that domains are allocated for a bus, rather than
an IOMMU instance.

Yes, Intel VT-d has this issue as well.  In theory we can have
heterogeneous IOMMU hardware units (DRHDs) in a system and the upper
bound of the geometry could be diminished if we add a less capable DRHD
into the domain.  I suspect this is more a theoretical problem than a
practical one though as we're typically mixing similar DRHDs and I think
we're still capable of 39-bit addressing in the least capable version
per the spec.

In any case, I really want to start testing geometry.force_aperture,
even if we're not yet comfortable to expose the IOMMU limits to the
user.  The vfio type1 shouldn't be enabled at all for underlying
hardware that allows DMA bypass.  Thanks,

Ok, I'll put it on my list of things to look at under the assumption that
the actual aperture limits don't need to be accurate as long as DMA to
an arbitrary unmapped address always faults.

I'm pretty sure we'd only ever set the aperture to the full input address range anyway (since we're not a GART-type thing), in which case we should only need to worry about unmatched streams that don't hit in a domain at all. Doesn't the disable_bypass option already cover that? (FWIW I hacked it up for v2 a while back, too[0]).



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