[cc +iommu]

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:
> > On Fri, 2015-10-16 at 16:03 +0200, Eric Auger wrote:
> > > Hi Alex,
> > > On 10/15/2015 10:52 PM, Alex Williamson wrote:
> > > > We can only provide isolation if DMA is forced through the IOMMU
> > > > aperture.  Don't allow type1 to be used if this is not the case.
> > > > 
> > > > Signed-off-by: Alex Williamson <alex.william...@redhat.com>
> > > > ---
> > > > 
> > > > Eric, I see a number of IOMMU drivers enable this, do the ones you
> > > > care about for ARM set geometry.force_aperture?  Thanks,
> > > I am currently using arm-smmu.c. I don't see force_aperture being set.
> > 
> > Hi Will,
> Hi Alex,
> > 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
> address.
> 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.

Hi Will,

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,


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