Unable to connect WinDbg to Hyper-V

I’m trying to use WinDbg 10 (1607) to debug Hyper-V and can’t get the target to connect.
* I have tried with Windows 2008 x64 and Windows 10 x64
* I checked my .vmx file and have my serial settings set to \.\pipe\com1_w2k (or win10) The other settings also look ok.
* I am following the directions on https://msdn.microsoft.com/en-us/library/windows/hardware/ff540654(v=vs.85).aspx
* vmdemux.exe prints “Requerying active channels from the hypervisor” Sometimes it also prints "Read Failed again 87071468
* kd.exe just shows “Waiting to reconnect…”
* On windows 2008, I was able to do echo hello>com1 and confirm from my host machine that the message was arriving in putty
* On Windows 10, I couldn’t echo to com1, but I could see that the early boot process is calling out because it dumps a chunk of non-ascii to COM1 followed by the text “hvi64.exe”
* In both cases, it was not connecting to remote.exe , kd.exe, or windbg.exe