linux-kernel - Re: [RFC PATCH] vfio/type1: Do not support IOMMUs that allow bypass

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Date:	Thu, 29 Oct 2015 18:28:20 +0000
From:	Will Deacon <will.deacon@....com>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	"eric.auger" <eric.auger@...aro.org>,
	iommu <iommu@...ts.linux-foundation.org>
Subject: Re: [RFC PATCH] vfio/type1: Do not support IOMMUs that allow bypass

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
> > 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.
> 
> 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.

Will
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