linux-kernel - Re: [PATCH] KVM: SVM: drop MSR_IA32_PERF

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Date:   Wed, 17 Jun 2020 09:47:01 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Like Xu <like.xu@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: SVM: drop MSR_IA32_PERF_CAPABILITIES from emulated MSRs

On Wed, Jun 17, 2020 at 4:38 AM Vitaly Kuznetsov <vkuznets@...hat.com> wrote:

> Side note: MSR_IA32_PERF_CAPABILITIES can be returned by both
> KVM_GET_MSR_INDEX_LIST and KVM_GET_MSR_FEATURE_INDEX_LIST as we have it
> both as an emulated MSR filtered by kvm_x86_ops.has_emulated_msr() and
> a feature msr filtered by kvm_x86_ops.get_msr_feature(). But the later
> is a whitelist so MSR_IA32_PERF_CAPABILITIES won't appear on AMD and the
> promise "can be passed to the KVM_GET_MSRS" is kept.

So, how is MSR_IA32_PERF_CAPABILITIES different from, say,
MSR_K7_HWCR, which by its very name doesn't sound like it would be
supported on Intel CPUs? Why not just emulate
MSR_IA32_PERF_CAPABILITIES on AMD, just as we emulate MSR_K7_HWCR on
Intel?

My concern is that we don't seem to have a standard here. Each
individual MSR is handled ad hoc, which adds unnecessary complexity.