Am 30.08.2018 um 20:53 schrieb Logan Gunthorpe:
+PCI Peer-to-Peer DMA Support
+The PCI bus has pretty decent support for performing DMA transfers
+between two devices on the bus. This type of transaction is henceforth
+called Peer-to-Peer (or P2P). However, there are a number of issues that
+make P2P transactions tricky to do in a perfectly safe way.
+One of the biggest issues is that PCI doesn't require forwarding
+transactions between hierarchy domains, and in PCIe, each Root Port
+defines a separate hierarchy domain. To make things worse, there is no
+simple way to determine if a given Root Complex supports this or not.
+(See PCIe r4.0, sec 1.3.1). Therefore, as of this writing, the kernel
+only supports doing P2P when the endpoints involved are all behind the
+same PCI bridge, as such devices are all in the same PCI hierarchy
+domain, and the spec guarantees that all transacations within the
+hierarchy will be routable, but it does not require routing
Can we add a kernel command line switch and a whitelist to enable P2P
between separate hierarchies?
At least all newer AMD chipsets supports this and I'm pretty sure that
Intel has a list with PCI-IDs of the root hubs for this as well.