On 04/01/18 08:33 PM, Alex Williamson wrote:

That's exactly what IOMMU groups represent, the smallest set of devices
which have DMA isolation from other devices.  By poking this hole, the
IOMMU group is invalid.  We cannot turn off ACS only for a specific
device, in order to enable p2p it needs to be disabled at every
downstream port between the devices where we want to enable p2p.
Depending on the topology, that could mean we're also enabling p2p for
unrelated devices.  Those unrelated devices might be in active use and
the p2p IOVAs now have a different destination which is no longer IOMMU
translated.

Oh, so IOMMU groups are created based on the existing hierarchy at boot 
time and not based on the user's needs for isolation?

Logan