Jerome and Christian
I think there is confusion here, Alex properly explained the scheme
PCIE-device do a ATS request to the IOMMU which returns a valid
translation for a virtual address. Device can then use that address
directly without going through IOMMU for translation.
So I went through ATS in version 4.0r1 of the PCI spec. It looks like even a ATS
translated TLP is still impacted by ACS though it has a separate control knob for
translated address TLPs (see 126.96.36.199 of 4.0r1 of the spec). So even if your device
supports ATS a P2P DMA will still be routed to the associated RP of the domain and down
again unless we disable ACS DT P2P on all bridges between the two devices involved in the
So we still don't get fine grained control with ATS and I guess we still have security
issues because a rogue or malfunctioning EP could just as easily issue TLPs with TA set vs
Also ATS is meaningless without something like PASID as far as i
ATS is still somewhat valuable without PSAID in the sense you can cache IOMMU address
translations at the EP. This saves hammering on the IOMMU as much in certain workloads.
Interestingly Section 188.8.131.52 almost mentions that Root Ports that support ATS AND can
implement P2P between root ports should advertise "ACS Direct Translated P2P
(T)" capability. This ties into the discussion around P2P between route ports we had
a few weeks ago...