On 23/03/18 03:50 PM, Bjorn Helgaas wrote:

Popping way up the stack, my original point was that I'm trying to
remove restrictions on what devices can participate in peer-to-peer
DMA.  I think it's fairly clear that in conventional PCI, any devices
in the same PCI hierarchy, i.e., below the same host-to-PCI bridge,
should be able to DMA to each other.

Yup, we are working on this.

The routing behavior of PCIe is supposed to be compatible with
conventional PCI, and I would argue that this effectively requires
multi-function PCIe devices to have the internal routing required to
avoid the route-to-self issue.

That would be very nice but many devices do not support the internal
route. We've had to work around this in the past and as I mentioned
earlier that NVMe devices have a flag indicating support. However, if a
device wants to be involved in P2P it must support it and we can exclude
devices that don't support it by simply not enabling their drivers.

Logan