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

[Jim Mattson]

On Mon, Mar 2, 2020 at 7:25 PM Jim Mattson <jmattson@xxxxxxxxxx> wrote:
>
> 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.

In fact, the more I think about it, the original change is correct, at
least in this regard. Your "fix" introduces undocumented and
unfathomable behavior.