Hector Martin

you can't do PCIe over USB because the former needs a hard worst-case latency guarantee and the latter explicitly avoids one

Wait, PCIe has latency guarantees? Didn't @marcan42 tunnel PCIe over some crazy-slow setup for PS4 stuff and have it work fine?

I believe yes, see PCIe Base Specification 3.5.2.1 and Table 3-4

I'm sure you could work around that if you can control the entire system, like by gating the TLP Transmitter's clock

ahahaha

or maaaaybe by terminating PCIe on either end and then using flow control to effectively stall the transmitter? no idea about PS4

You do not tunnel the data link layer. Those timeouts are for TLP transmission acks, not TLP to completion (which is what we care about).

For comparison, USB has tight reply ACK timing requirements too (but nobody who isn't writing a USB device controller has to deal with that)