From mboxrd@z Thu Jan  1 00:00:00 1970
From: Wei Wang <wei.w.wang@intel.com>
Subject: Re: [PATCH v4 05/10] KVM/x86: expose MSR_IA32_PERF_CAPABILITIES to
 the guest
Date: Mon, 07 Jan 2019 17:15:13 +0800
Message-ID: <5C3318A1.9090009@intel.com>
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>
Mime-Version: 1.0
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
Cc: LKML <linux-kernel@vger.kernel.org>,
        kvm list <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Andi Kleen <ak@linux.intel.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
To: Jim Mattson <jmattson@google.com>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <CALMp9eQ0nzKBP7az72G+eRe8C4KQQUBdj+7oD_WJHSFfYkj+Vw@mail.gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: kvm.vger.kernel.org

On 01/03/2019 11:25 PM, Jim Mattson wrote:
> On Wed, Jan 2, 2019 at 11:55 PM Wei Wang <wei.w.wang@intel.com> wrote:
>
>> Right, thanks. Probably better to change it to below:
>>
>> msr_info->data = 0;
>> data = native_read_msr(MSR_IA32_PERF_CAPABILITIES);
>> if (vcpu->kvm->arch.lbr_in_guest)
>>       msr_info->data |= (data & X86_PERF_CAP_MASK_LBR_FMT);
>>
> This still breaks backwards compatibility. Returning 0 and raising #GP
> are not the same.

I'm not sure about raising GP# in this case.

This PERF_CAP msr contains more things than the lbr format.
For example, a guest with lbr=false option could read it to get PEBS_FMT,
which is PERF_CAP[11:8]. We should offer those bits in this case.

When lbr=false, the lbr feature is not usable by the guest,
so I think whatever value (0 or other value) of the LBR_FMT bits that
we give to the guest might not be important.

Best,
Wei