[PATCH v4 04/14] PCI/P2PDMA: Clear ACS P2P flags for all devices behind switches

[Don Dutile]

On 05/09/2018 10:44 AM, Alex Williamson wrote:
> On Wed, 9 May 2018 12:35:56 +0000
> "Stephen  Bates" <sbates at raithlin.com> wrote:
> 
>> Hi Alex and Don
>>
>>>     Correct, the VM has no concept of the host's IOMMU groups, only
>>> the hypervisor knows about the groups,
>>
>> But as I understand it these groups are usually passed through to VMs
>> on a pre-group basis by the hypervisor? So IOMMU group 1 might be
>> passed to VM A and IOMMU group 2 passed to VM B. So I agree the VM is
>> not aware of IOMMU groupings but it is impacted by them in the sense
>> that if the groupings change the PCI topology presented to the VM
>> needs to change too.
> 
> Hypervisors don't currently expose any topology based on the grouping,
> the only case where such a concept even makes sense is when a vIOMMU is
> present as devices within the same group cannot have separate address
> spaces.  Our options for exposing such information is also limited, our
> only real option would seem to be placing devices within the same group
> together on a conventional PCI bus to denote the address space
> granularity.  Currently we strongly recommend singleton groups for this
> case and leave any additional configuration constraints to the admin.
> 
> The case you note of a group passed to VM A and another passed to VM B
> is exactly an example of why any sort of dynamic routing change needs to
> have the groups fully released, such as via hot-unplug.  For instance,
> a routing change at a shared node above groups 1 & 2 could result in
> the merging of these groups and there is absolutely no way to handle
> that with portions of the group being owned by two separate VMs after
> the merge.  Thanks,
> 
> Alex
> 
The above is why I stated the host/HV has to do p2p setup *before* device-assignment
is done.
Now, that could be done at boot time (with a mod.conf-like config in host/HV, before VM startup)
as well.
Dynamically, if such a feature is needed, requires a hot-unplug/plug cycling as Alex states.