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Date:   Mon, 7 Jan 2019 10:20:12 -0800
From:   Andi Kleen <ak@linux.intel.com>
To:     Jim Mattson <jmattson@google.com>
Cc:     Wei Wang <wei.w.wang@intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
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        arei.gonglei@huawei.com
Subject: Re: [PATCH v4 05/10] KVM/x86: expose MSR_IA32_PERF_CAPABILITIES to
 the guest
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> The issue is compatibility. Prior to your change, reading this MSR
> from a VM would raise #GP. After your change, it won't. That means
> that if you have a VM migrating between hosts with kernel versions
> before and after this change, the results will be inconsistent. In the

No it will not be. All Linux kernel uses of this MSR are guarded
by a CPUID check.

-Andi