We're able to successfully attach using a 4.14 kernel but 5.4 and 5.6 kernels fail the handshake during attach. The kgdboe module seems to load fine on the 5.x kernels and gdb triggers the breakpoint on the target node but the following messages appears in gdb:

Remote replied unexpectedly to 'vMustReplyEmpty': timeout

This leaves the target node hung because it's at the kgdb breakpoint and we can't issue the command to resume.