If I reboot to Windows directly from Windows (bootcamp options), I cannot launch a vm in Hyper-V

You should probably ask that in the Boot Camp forum. If you would like I can ask for the question to be moved to that forum.

You should always use System Preferences -> Startup Disk. It enables the CSM-BIOS layer. When you use the Option Key, Windows will come up, but underlying CSM-BIOS layer is not invoked. The Hyper-V depends on BIOS settings, it will only work when using the Startup Disk option.

Also see man bless and --legacydrivehint qualifier and https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/ man8/bless.8.html.

Thanks for posting back with your test results.

I just tried this on a 2015 rMBP and the bless command fails with a message "legacy mode is not supported on this system". Any idea what that means, am I out of luck?

The 2015 Macs will not support CSM-BIOS correctly. If you really want this functionality, find a 2013/2014 Mac model.

I have a 2015 MacBook 12" and I am just now reading these post after experiencing similar issues with using hyper-v. I hope to inform others and maybe talk about a fix. I followed the recommendations of powering completely down launching into osx and restarting to windows, as my startup disk selection was already windows. After restarting I experience a blank screen with a movable cursor. Only way out is a hard shutdown. Next boot Windows 10 loads just fine, but system information indicates hyper-v virtualization enabled in firmware "No." I have repeated this blank screen numerous times. Loading Windows properly only once with the ability to run a VM after a pram reset. Never again after restarting that session no matter what I did. I have tried the legacy set command and it fails. All drivers are up to date.

Loner T - I think it's clear your statement 2015 Macs will not support CSM-BIOS is correct. Why is that? Thanks in advance for thinking about this problem.

Apple choose to deprecate support for legacy BIOS/MBR OSes (notably W7). W8+ and W10 are using EFI on Macs, so there is no real need for CSM-BIOS. Apple should have allowed an equivalent EFI supported method for Hyper-V, but apparently it is not there.

Do you anticipate a fix in the firmware, as it seems everything hardware is there for hyper-v?

My suggestion is to create a bug report - https://bugreport.apple.com/problem/viewproblem and have the Apple Engineers follow it up. Usually they are good at follow-up and at providing updates via Software Updates/App Store.

Loner T I followed your advice and created a bug report back in December. Just curious if you have found a work around or know of an update on the issue? Thanks for your diligence in these forms.