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References: <20200302195736.24777-1-sean.j.christopherson@intel.com> <20200302195736.24777-3-sean.j.christopherson@intel.com>
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From:   Jim Mattson <jmattson@google.com>
Date:   Mon, 2 Mar 2020 19:25:43 -0800
Message-ID: <CALMp9eThBnN3ktAfwhNs7L-O031JDFqjb67OMPooGvmkcdhK4A@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 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.