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

[Robin Murphy <robin.murphy@arm.com>]

On 08/12/16 17:01, Alex Williamson wrote:
> On Thu, 8 Dec 2016 13:14:04 +0000
> Robin Murphy <robin.murphy@arm.com> wrote:
>> On 08/12/16 09:36, Auger Eric wrote:
>>> 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.
> 
> Eric and I talked about this and I don't see the value in identifying
> an RMRR as anything other than a reserved range for a device.  It's not
> userspace's job to maintain an identify mapped range for the device,
> and it can't be trusted to do so anyway.  It does throw a kink in the
> machinery though as an RMRR is a reserved memory range unique to a
> device.  It doesn't really fit into a monolithic /sys/class/iommu view
> of global reserved regions as an RMRR is only relevant to the device
> paths affected.

I think we're in violent agreement then - to clarify, I was thinking in
terms of patch 7 of this series, where everything relevant to a
particular group would be exposed as just an opaque "don't use this
address range" regardless of the internal type.

I'm less convinced the kernel has any need to provide its own 'global'
view of reservations which strictly are always at least per-IOMMU, if
not per-root-complex, even when all the instances do share the same
address by design. The group-based interface fits the reality neatly,
and userspace can easily iterate all the groups if it wants to consider
everything. Plus if it doesn't want to, then it needn't bother reserving
anything which doesn't apply to the group(s) it's going to bind to VFIO.

Robin.

> Another kink is that sometimes we know what the RMRR is for, know that
> it's irrelevant for our use case, and ignore it.  This is true for USB
> and Intel graphics use cases of RMRRs.
> 
> Also, aside from the above mentioned cases, devices with RMRRs are
> currently excluded from participating in the IOMMU API by the
> intel-iommu driver and I expect this to continue in the general case
> regardless of whether the ranges are more easily exposed to userspace.
> ARM may have to deal with mangling a guest memory map due to lack of
> any standard layout, de facto or otherwise, but for x86 I don't think
> it's worth the migration and hotplug implications.  Thanks,
> 
> Alex
>