Enoch boot hangs when KVM switch present

[RePete222]

I recently installed a USB/DVI KVM switch to allow my AMD hack (see sig) to share a monitor, KB & mouse with my work laptop.  In order to boot successfully, I have had to disable legacy USB in the BIOS, which makes it impossible to enter any boot flag/boot volume overrides.  Once booting is complete, the keyboard works fine.  If I leave legacy USB enabled in the BIOS, booting hangs after the "EHCI controller ..." message is displayed.  What combination of boot flags and/or kexts do I need to allow legacy USB support and the KVM switch to play nice during startup?

I am also testing Enoch Rev 2908 in preparation for upgrading from El Cap to High Sierra, and am having similar issues with that version (booting to an El Cap test partition).  And, when booting with legacy USB disabled, the screen goes dark at the point of changing resolution, and the machine shuts down after a few seconds.

Any help on these issues would be most appreciated.

Thanks.

[RePete222]

UPDATE:  Last night, after reading some other posts (dealing more with legacy USB than KVM switches), I got the idea to try the combination of Legacy USB Enabled and USB Bus Speed FullSpeed (as opposed to HiSpeed) BIOS settings.  It worked!  The keyboard was able to enter boot overrides, and the system booted up all the way, with KB/mouse working fine at the desktop.

My one concern with this workaround is: Will all USB ports be dumbed-down to USB 1 speed on the running system?  Or does that only apply to the boot phase, or to ports that were in use at boot time?  What is it about the KVM switch that would require this, when it has not been necessary with no KVM switch present?

I have not had a chance to try this workaround with Enoch Rev 2908 yet; will have to try it this evening.

[RePete222]

Well, attempting to boot with Enoch Rev 2908 under the latest BIOS settings results in either hanging at [PCI configuration begin] or shutting down about 5 seconds after the screen goes blank to switch resolution.

EDIT:  Partial success--when booting with Rev 2908 to live boot partition, screen goes dark at point of resolution switch, but booting continues, and eventually reaches the desktop.  So something not working with backend verbose-mode messages getting to the screen.  I get the shutdown 5 seconds after resolution switch when booting to a test partition (both boot partitions are El Cap).

Edited April 18, 2018 by RePete222
Partial success update