linux-kernel - Re: [PATCH v4 05/10] KVM/x86: expose MSR_IA32_PERF

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Message-ID: <CALMp9eQ0nzKBP7az72G+eRe8C4KQQUBdj+7oD_WJHSFfYkj+Vw@mail.gmail.com>
Date:   Thu, 3 Jan 2019 07:25:55 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Kan Liang <kan.liang@...el.com>,
        Ingo Molnar <mingo@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        like.xu@...el.com, Jann Horn <jannh@...gle.com>,
        arei.gonglei@...wei.com
Subject: Re: [PATCH v4 05/10] KVM/x86: expose MSR_IA32_PERF_CAPABILITIES to
 the guest

On Wed, Jan 2, 2019 at 11:55 PM Wei Wang <wei.w.wang@...el.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.