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

[Don Dutile <ddutile@redhat.com>]

On 05/08/2018 08:01 PM, Alex Williamson wrote:
> On Tue, 8 May 2018 19:06:17 -0400
> Don Dutile <ddutile@redhat.com> wrote:
>> On 05/08/2018 05:27 PM, Stephen  Bates wrote:
>>> As I understand it VMs need to know because VFIO passes IOMMU
>>> grouping up into the VMs. So if a IOMMU grouping changes the VM's
>>> view of its PCIe topology changes. I think we even have to be
>>> cognizant of the fact the OS running on the VM may not even support
>>> hot-plug of PCI devices.
>> Alex:
>> Really? IOMMU groups are created by the kernel, so don't know how
>> they would be passed into the VMs, unless indirectly via  PCI(e)
>> layout. At best, twiddling w/ACS enablement (emulation) would cause
>> VMs to see different IOMMU groups, but again, VMs are not the
>> security point/level, the host/HV's are.
> 
> Correct, the VM has no concept of the host's IOMMU groups, only the
> hypervisor knows about the groups, but really only to the extent of
> which device belongs to which group and whether the group is viable.
> Any runtime change to grouping though would require DMA mapping
> updates, which I don't see how we can reasonably do with drivers,
> vfio-pci or native host drivers, bound to the affected devices.  Thanks,
> 
> Alex
> 
A change in iommu groups would/could require a device remove/add cycle to get an updated DMA-mapping (yet-another-overused-term: iommu 'domain').