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From:   Jim Mattson <jmattson@google.com>
Date:   Mon, 2 Mar 2020 20:25:31 -0800
Message-ID: <CALMp9eR0Mw8iPv_Z43gfCEbErHQ6EXX8oghJJb5Xge+47ZU9yQ@mail.gmail.com>
Subject: Re: [PATCH 2/6] KVM: x86: Fix CPUID range check for Centaur and
 Hypervisor ranges
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Joerg Roedel <joro@8bytes.org>, kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Jan Kiszka <jan.kiszka@siemens.com>,
        Xiaoyao Li <xiaoyao.li@intel.com>
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On Mon, Mar 2, 2020 at 7:25 PM Jim Mattson <jmattson@google.com> wrote:
>
> On Mon, Mar 2, 2020 at 11:57 AM Sean Christopherson
> <sean.j.christopherson@intel.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.

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.