Re: Nested kvm_intel broken on pre 3.3 hosts

[Nadav Har'El <nyh@math.technion.ac.il>]

On Wed, Aug 01, 2012, Avi Kivity wrote about "Re: Nested kvm_intel broken on pre 3.3 hosts":
> Right - it's not just kvm-as-a-guest that will trip on this.  But
> there's no point in everyone backporting it on their own.  If you're
> doing the backport, please post it here and we'll forward it to the
> stable branch.

If I understand correctly, the failure occurs because new versions of
KVM refuse to work if the processor doesn't support CPU_BASED_RDPMC_EXITING -
which older versions of nested VMX didn't say that they did.

But must the KVM guest refuse to work if this feature isn't supported?
I.e., why not move in setup_vmcs_config() the CPU_BASED_RDPMC_EXITING
from "min" to "opt"? Isn't losing the PMU feature a lesser evil than
not working at all?  In any case, perhaps the original reporter can use
this as a workaround, at least, because it requires modifying the (L1)
guest, not the host.


-- 
Nadav Har'El                        |        Wednesday, Aug 1 2012, 13 Av 5772
nyh@math.technion.ac.il             |-----------------------------------------
Phone +972-523-790466, ICQ 13349191 |May you live as long as you want - and
http://nadav.harel.org.il           |never want as long as you live.