64 bit mode broken in Ubuntu 18.04 LTS (KVM_SET_SREGS: Invalid argument)

[dpw]

64 bit mode broken in Ubuntu 18.04 LTS (KVM_SET_SREGS: Invalid argument)

[AnonymousII]

Update (fixed): After some further experiments I'm pretty sure to have fixed the problem:
In the EFER-Control Register the bit EFER_LMA = (1U << 10) (=Long Mode Active, see https://en.wikipedia.org/wiki/Control_register#EFER) needs to be set.

Obviously the old KVM version did not check for "Long Mode Active" to be set and was satisfied with "Long Mode Enable".
So Adding the following line of code to kvm-hello-world.c:setup_long_mode() obviously fixes the problem:
sregs->efer |= (1U << 10);//EFER_LMA

Would be cool if you could patch the code so that it works on the latest Ubuntu version in case others run into the same problem :)

Anyway I don't understand why there's two different flags "enable" and "active" for long mode:
Why would it make sense to enable but not activate long mode or to activate long mode without it being enabled? I'm a bit confused why there's two different control bits here... Can you maybe tell me why there's two "different" flags for long mode?

Kind regards,
Andreas

[justinc1]

The 64 bit code crashed on Fedora 27 too. The patch from @AnonymousII helped. I took liberty to send pull request (but it has wrong signed off header).

[dpw]

Thanks. Sorry it took a while for me to merge the fix - I needed to reinstall a machine at home to test.