linux-kernel - Re: [PATCH v6 06/16] KVM: x86/pmu: Add IA32_PEBS

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Date:   Tue, 18 May 2021 15:42:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Xu, Like" <like.xu@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, weijiang.yang@...el.com,
        Kan Liang <kan.liang@...ux.intel.com>, ak@...ux.intel.com,
        wei.w.wang@...el.com, eranian@...gle.com, liuxiangdong5@...wei.com,
        linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
        Luwei Kang <luwei.kang@...el.com>,
        Like Xu <like.xu@...ux.intel.com>
Subject: Re: [PATCH v6 06/16] KVM: x86/pmu: Add IA32_PEBS_ENABLE MSR
 emulation for extended PEBS

On Tue, May 18, 2021 at 04:44:13PM +0800, Xu, Like wrote:
> Will adding the following comments help you ?
> 
> +/*
> + * Currently, the only caller of this function is the atomic_switch_perf_msrs().
> + * The host perf conext helps to prepare the values of the real hardware for
> + * a set of msrs that need to be switched atomically in a vmx transaction.
> + *
> + * For example, the pseudocode needed to add a new msr should look like:
> + *
> + * arr[(*nr)++] = (struct perf_guest_switch_msr){
> + *     .msr = the hardware msr address,
> + *     .host = the value the hardware has when it doesn't run a guest,
> + *     .guest = the value the hardware has when it runs a guest,

So personally I think the .host and .guest naming is terrible here,
because both values are host values. But I don't know enough about virt
to know if there's accepted nomencature for this.

> + * };
> + *
> + * These values have nothing to do with the emulated values the guest sees
> + * when it uses {RD,WR}MSR, which should be handled in the KVM context.
> + */
>  static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr, void *data)

Yes, now at least one can understand wth this function does, even though
the actual naming is still horrible. Thanks!

Additionally, would it make sense to add a pointer to the KVM code that
does the emulation for each MSR listed in this function?