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

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Date:	Tue, 27 Oct 2015 15:40:44 +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>
Subject: Re: [RFC PATCH] vfio/type1: Do not support IOMMUs that allow bypass

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.williamson@...hat.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.

Will
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