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

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  Re: [RFC PATCH] vfio/type1: Do not support IOMMUs that allow bypass Will Deacon <will.deacon@arm.com> - 2015-10-27 16:50 +0100
    Re: [RFC PATCH] vfio/type1: Do not support IOMMUs that allow bypass Alex Williamson <alex.williamson@redhat.com> - 2015-10-27 17:10 +0100
      Re: [RFC PATCH] vfio/type1: Do not support IOMMUs that allow bypass Will Deacon <will.deacon@arm.com> - 2015-10-29 19:30 +0100
        Re: [RFC PATCH] vfio/type1: Do not support IOMMUs that allow bypass Robin Murphy <robin.murphy@arm.com> - 2015-10-29 19:50 +0100
          Re: [RFC PATCH] vfio/type1: Do not support IOMMUs that allow bypass Will Deacon <will.deacon@arm.com> - 2015-10-29 20:00 +0100

#1256862 — 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@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.

Will
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#1256893

[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.williamson@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,

Alex

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#1258945

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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#1258951

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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#1258953

On Thu, Oct 29, 2015 at 06:42:10PM +0000, Robin Murphy wrote:
> 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]).

Well, the "full input address range" is tricky when you have multiple SMMU
instances with different input address sizes. I can do something similar
to the pgsize_bitmap.

I also don't think the disable_bypass option is what we're after -- this
is about devices attached to a VFIO domain that can still mysteriously
bypass the SMMU for some ranges AFAIU (and shouldn't be an issue for ARM).

Will
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