Re: [RFC v3 00/10] KVM PCIe/MSI passthrough on ARM/ARM64 and IOVA reserved regions

[Auger Eric]

Hi Robin,

On 08/12/2016 14:14, Robin Murphy wrote:
> On 08/12/16 09:36, Auger Eric wrote:
>> Hi,
>>
>> On 15/11/2016 14:09, Eric Auger wrote:
>>> Following LPC discussions, we now report reserved regions through
>>> iommu-group sysfs reserved_regions attribute file.
>>
>>
>> While I am respinning this series into v4, here is a tentative summary
>> of technical topics for which no consensus was reached at this point.
>>
>> 1) Shall we report the usable IOVA range instead of reserved IOVA
>>    ranges. Not discussed at/after LPC.
>>    x I currently report reserved regions. Alex expressed the need to
>>      report the full usable IOVA range instead (x86 min-max range
>>      minus MSI APIC window). I think this is meaningful for ARM
>>      too where arm-smmu might not support the full 64b range.
>>    x Any objection we report the usable IOVA regions instead?
> 
> The issue with that is that we can't actually report "the usable
> regions" at the moment, as that involves pulling together disjoint
> properties of arbitrary hardware unrelated to the IOMMU. We'd be
> reporting "the not-definitely-unusable regions, which may have some
> unusable holes in them still". That seems like an ABI nightmare - I'd
> still much rather say "here are some, but not necessarily all, regions
> you definitely can't use", because saying "here are some regions which
> you might be able to use most of, probably" is what we're already doing
> today, via a single implicit region from 0 to ULONG_MAX ;)
> 
> The address space limits are definitely useful to know, but I think it
> would be better to expose them separately to avoid the ambiguity. At
> worst, I guess it would be reasonable to express the limits via an
> "out-of-range" reserved region type for 0 to $base and $top to
> ULONG-MAX. To *safely* expose usable regions, we'd have to start out
> with a very conservative assumption (e.g. only IOVAs matching physical
> RAM), and only expand them once we're sure we can detect every possible
> bit of problematic hardware in the system - that's just too limiting to
> be useful. And if we expose something knowingly inaccurate, we risk
> having another "bogoMIPS in /proc/cpuinfo" ABI burden on our hands, and
> nobody wants that...
Makes sense to me. "out-of-range reserved region type for 0 to $base and
$top to ULONG-MAX" can be an alternative to fulfill the requirement.
> 
>> 2) Shall the kernel check collision with MSI window* when userspace
>>    calls VFIO_IOMMU_MAP_DMA?
>>    Joerg/Will No; Alex yes
>>    *for IOVA regions consumed downstream to the IOMMU: everyone says NO
> 
> If we're starting off by having the SMMU drivers expose it as a fake
> fixed region, I don't think we need to worry about this yet. We all seem
> to agree that as long as we communicate the fixed regions to userspace,
> it's then userspace's job to work around them. Let's come back to this
> one once we actually get to the point of dynamically sizing and
> allocating 'real' MSI remapping region(s).
> 
> Ultimately, the kernel *will* police collisions either way, because an
> underlying iommu_map() is going to fail if overlapping IOVAs are ever
> actually used, so it's really just a question of whether to have a more
> user-friendly failure mode.
That's true on ARM but not on x86 where the APIC MSI region is not
mapped I think.
> 
>> 3) RMRR reporting in the iommu group sysfs? Joerg: yes; Don: no
>>    My current series does not expose them in iommu group sysfs.
>>    I understand we can expose the RMRR regions in the iomm group sysfs
>>    without necessarily supporting RMRR requiring device assignment.
>>    We can also add this support later.
> 
> As you say, reporting them doesn't necessitate allowing device
> assignment, and it's information which can already be easily grovelled
> out of dmesg (for intel-iommu at least) - there doesn't seem to be any
> need to hide them, but the x86 folks can have the final word on that.
agreed

Thanks

Eric
> 
> Robin.
> 
>> Thanks
>>
>> Eric
>>
>>
>>>
>>> Reserved regions are populated through the IOMMU get_resv_region callback
>>> (former get_dm_regions), now implemented by amd-iommu, intel-iommu and
>>> arm-smmu.
>>>
>>> The intel-iommu reports the [FEE0_0000h - FEF0_000h] MSI window as an
>>> IOMMU_RESV_NOMAP reserved region.
>>>
>>> arm-smmu reports the MSI window (arbitrarily located at 0x8000000 and
>>> 1MB large) and the PCI host bridge windows.
>>>
>>> The series integrates a not officially posted patch from Robin:
>>> "iommu/dma: Allow MSI-only cookies".
>>>
>>> This series currently does not address IRQ safety assessment.
>>>
>>> Best Regards
>>>
>>> Eric
>>>
>>> Git: complete series available at
>>> https://github.com/eauger/linux/tree/v4.9-rc5-reserved-rfc-v3
>>>
>>> History:
>>> RFC v2 -> v3:
>>> - switch to an iommu-group sysfs API
>>> - use new dummy allocator provided by Robin
>>> - dummy allocator initialized by vfio-iommu-type1 after enumerating
>>>   the reserved regions
>>> - at the moment ARM MSI base address/size is left unchanged compared
>>>   to v2
>>> - we currently report reserved regions and not usable IOVA regions as
>>>   requested by Alex
>>>
>>> RFC v1 -> v2:
>>> - fix intel_add_reserved_regions
>>> - add mutex lock/unlock in vfio_iommu_type1
>>>
>>>
>>> Eric Auger (10):
>>>   iommu/dma: Allow MSI-only cookies
>>>   iommu: Rename iommu_dm_regions into iommu_resv_regions
>>>   iommu: Add new reserved IOMMU attributes
>>>   iommu: iommu_alloc_resv_region
>>>   iommu: Do not map reserved regions
>>>   iommu: iommu_get_group_resv_regions
>>>   iommu: Implement reserved_regions iommu-group sysfs file
>>>   iommu/vt-d: Implement reserved region get/put callbacks
>>>   iommu/arm-smmu: Implement reserved region get/put callbacks
>>>   vfio/type1: Get MSI cookie
>>>
>>>  drivers/iommu/amd_iommu.c       |  20 +++---
>>>  drivers/iommu/arm-smmu.c        |  52 +++++++++++++++
>>>  drivers/iommu/dma-iommu.c       | 116 ++++++++++++++++++++++++++-------
>>>  drivers/iommu/intel-iommu.c     |  50 ++++++++++----
>>>  drivers/iommu/iommu.c           | 141 ++++++++++++++++++++++++++++++++++++----
>>>  drivers/vfio/vfio_iommu_type1.c |  26 ++++++++
>>>  include/linux/dma-iommu.h       |   7 ++
>>>  include/linux/iommu.h           |  49 ++++++++++----
>>>  8 files changed, 391 insertions(+), 70 deletions(-)
>>>
> 
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