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

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Date:   Mon, 2 Mar 2020 19:25:43 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jan Kiszka <jan.kiszka@...mens.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>
Subject: Re: [PATCH 2/6] KVM: x86: Fix CPUID range check for Centaur and
 Hypervisor ranges

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