Comment 7 for bug 1031090

On 08/01/2012 06:07 PM, Nadav Har'El wrote:
> 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.

Right.

> 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.

Real processors that don't support RDPMC exiting don't exist (and
logically cannot exist unless they also drop support for the RDPMC
instruction). Given it's a clear host bug I'd rather fix it than making
the guest more complicated, even by a small amount.

The hypervisor needs to be updated on a regular schedule anyway, so
there's no risk of locking users out for more than a short while.