Re: [PATCH 2/6] KVM: x86: Fix CPUID range check for Centaur and Hypervisor ranges

[Jim Mattson]

On Mon, Mar 2, 2020 at 11:57 AM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:

> The bad behavior can be visually confirmed by dumping CPUID output in
> the guest when running Qemu with a stable TSC, as Qemu extends the limit
> of range 0x40000000 to 0x40000010 to advertise VMware's cpuid_freq,
> without defining zeroed entries for 0x40000002 - 0x4000000f.

I think it could be reasonably argued that this is a userspace bug.
Clearly, when userspace explicitly supplies the results for a leaf,
those results override the default CPUID values for that leaf. But I
haven't seen it documented anywhere that leaves *not* explicitly
supplied by userspace will override the default CPUID values, just
because they happen to appear in some magic range.