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Message-ID: <5C34570D.6010208@intel.com>
Date:   Tue, 08 Jan 2019 15:53:49 +0800
From:   Wei Wang <wei.w.wang@intel.com>
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To:     Jim Mattson <jmattson@google.com>, Andi Kleen <ak@linux.intel.com>
CC:     LKML <linux-kernel@vger.kernel.org>,
        kvm list <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Kan Liang <kan.liang@intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        like.xu@intel.com, Jann Horn <jannh@google.com>,
        arei.gonglei@huawei.com
Subject: Re: [PATCH v4 05/10] KVM/x86: expose MSR_IA32_PERF_CAPABILITIES to
 the guest
References: <1545816338-1171-1-git-send-email-wei.w.wang@intel.com> <1545816338-1171-6-git-send-email-wei.w.wang@intel.com> <CALMp9eTOV3Twep9gL-9S+Pe_k-=v17CcJTLb5=+7_pjvWf9RfQ@mail.gmail.com> <5C2DC132.9050103@intel.com> <CALMp9eQ0nzKBP7az72G+eRe8C4KQQUBdj+7oD_WJHSFfYkj+Vw@mail.gmail.com> <5C3318A1.9090009@intel.com> <CALMp9eSyC5arXkrFdJKCx=cG7uF6tVgkbSJbUJArf4cFje13rQ@mail.gmail.com> <20190107182012.GL6118@tassilo.jf.intel.com> <CALMp9eT2J=qFTEEpsym_aqeCRwyCquTYnY8=taZ3nY3rs8WeJA@mail.gmail.com>
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On 01/08/2019 02:48 AM, Jim Mattson wrote:
> On Mon, Jan 7, 2019 at 10:20 AM Andi Kleen <ak@linux.intel.com> wrote:
>>> 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.
> Linux usage is irrelevant to the architected behavior of the virtual
> CPU. According to volume 4 of the SDM, this MSR is only supported when
> CPUID.01H:ECX.PDCM [bit 15] is set. Therefore, kvm should raise #GP
> whenever a guest tries to read this MSR and the guest's
> CPUID.01H:ECX.PDCM [bit 15] is clear.
>

Probably one more check would be better:

if (!boot_cpu_has(X86_FEATURE_PDCM) ||
     !guest_cpuid_has(vcpu, X86_FEATURE_PDCM))
     return 1;

(host isn't expected to read this MSR when PDCM is not supported
by the guest, so don't have "!msr_info->host_initiate" added to above)

Best,
Wei