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

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Message-ID: <56326882.10109@arm.com>
Date:	Thu, 29 Oct 2015 18:42:10 +0000
From:	Robin Murphy <robin.murphy@....com>
To:	Will Deacon <will.deacon@....com>,
	Alex Williamson <alex.williamson@...hat.com>
Cc:	iommu <iommu@...ts.linux-foundation.org>,
	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 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
>>> 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.

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]).

Robin.

[0]:http://www.linux-arm.org/git?p=linux-rm.git;a=commitdiff;h=23a251189fa3330b799a837bd8eb1023aa2dcea4

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