Almost all communications between devices is memory mapped i/o these days. x86 has i/o ports, but that's mostly used for devices that were specified long ago (although some things are still relevant).

If there was a core to core bulk messaging system, it would be memory mapped as well, and you'd need to store the messages in some sort of memory, so why add a specialized message queue, when you can just use memory?

You can do things like add a message to a mailbox in memory, and then send an interrupt to the other processor to indicate it's ready.