But it would be a much easier proposal to disable ACS when the
not enabled, ACS has no real purpose in that case.
I guess one issue I have with this is that it disables IOMMU groups for all Root Ports and
not just the one(s) we wish to do p2pdma on.
The IOMMU and P2P are already not exclusive, we can bounce off
IOMMU or make use of ATS as we've previously discussed. We were
previously talking about a build time config option that you didn't
expect distros to use, so I don't think intervention for the user to
disable the IOMMU if it's enabled by default is a serious concern
ATS definitely makes things more interesting for the cases where the EPs support it.
However I don't really have a handle on how common ATS support is going to be in the
kinds of devices we have been focused on (NVMe SSDs and RDMA NICs mostly).
What you're trying to do is enabled direct peer-to-peer for
which do not support ATS when the IOMMU is enabled, which is not
something that necessarily makes sense to me.
As above the advantage of leaving the IOMMU on is that it allows for both p2pdma PCI
domains and IOMMU groupings PCI domains in the same system. It is just that these domains
will be separate to each other.
So that leaves avoiding bounce buffers as the remaining IOMMU
I agree with you here that the devices we will want to use for p2p will probably not
require a bounce buffer and will support 64 bit DMA addressing.
I'm still not seeing why it's terribly undesirable to require
devices to support
ATS if they want to do direct P2P with an IOMMU enabled.
I think the one reason is for the use-case above. Allowing IOMMU groupings on one domain
and p2pdma on another domain....