On 08/05/18 01:34 PM, Alex Williamson wrote: > They are not so unrelated, see the ACS Direct Translated P2P > capability, which in fact must be implemented by switch downstream > ports implementing ACS and works specifically with ATS. This appears to > be the way the PCI SIG would intend for P2P to occur within an IOMMU > managed topology, routing pre-translated DMA directly between peer > devices while requiring non-translated requests to bounce through the > IOMMU. Really, what's the value of having an I/O virtual address space > provided by an IOMMU if we're going to allow physical DMA between > downstream devices, couldn't we just turn off the IOMMU altogether? Of > course ATS is not without holes itself, basically that we trust the > endpoint's implementation of ATS implicitly. Thanks, I agree that this is what the SIG intends, but I don't think hardware fully supports this methodology yet. The Direct Translated capability just requires switches to forward packets that have the AT request type set. It does not require them to do the translation or to support ATS such that P2P requests can be translated by the IOMMU. I expect this is so that an downstream device can implement ATS and not get messed up by an upstream switch that doesn't support it. Logan
