On 01/03/18 02:21 PM, Alex Williamson wrote:
This is still a pretty terrible solution though, your kernel
needs to decide whether they favor device assignment or p2p, because we
can't do both, unless there's a patch I haven't seen yet that allows
boot time rather than compile time configuration. There are absolutely
supported device assignment cases of switches proving isolation between
devices allowing the downstream EPs to be used independently. I think
this is a non-starter for distribution support without boot time or
dynamic configuration. I could imagine dynamic configuration through
sysfs that might trigger a soft remove and rescan of the affected
devices in order to rebuild the IOMMU group. The hard part might be
determining which points to allow that to guarantee correctness. For
instance, upstream switch ports don't actually support ACS, but they'd
otherwise be an obvious concentration point to trigger a
At this point, I don't expect this to be enabled in any distribution.
The use case is for custom hardware intended to do P2P which means they
can have a custom kernel with P2P enabled. The default for
CONFIG_PCI_P2P is 'no' and I expect it to stay that way for a long time.
Another boot option which has to be set for this to work isn't something
we'd like to see.