Am 30.08.2018 um 20:53 schrieb Logan Gunthorpe:
[SNIP] +============================ +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 +between hierarchies.
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.
Regards, Christian.
