Re: [PATCH v2 04/10] PCI/P2PDMA: Clear ACS P2P flags for all devices behind switches

[Logan Gunthorpe]

On 01/03/18 02:21 PM, Alex Williamson wrote:
> This is still a pretty terrible solution though, your kernel provider
> 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
> reconfiguration.  Thanks,

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.

Logan