On 14/03/18 06:16 AM, David Laight wrote:

That surprises me (unless I missed something last time I read the spec).
While P2P writes are relatively easy to handle, reads and any other TLP that
require acks are a completely different proposition.
There are no additional fields that can be set in the read TLP and will be
reflected back in the ack(s) than can be used to route the acks back to the
correct initiator.

I'm pretty sure that to support P2P reads a switch would have to save
the received read TLP and (possibly later on) issue read TLP of its own
for the required data.
I'm not even sure it is easy to interleave the P2P reads with those
coming from the root.
That requires a potentially infinite queue of pending requests.

This is wrong. A completion is a TLP just like any other and makes use
of the Destination ID field in the header to route it back to the
original requester.

Some x86 root ports support P2P writes (maybe with a bios option).
It would be a shame not to be able to do P2P writes on such systems
even though P2P reads won't work.

Yes, and this has been discussed many times. It won't be changing in the
near term.

Logan