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.
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