linux-kernel - Re: [PATCH v2 2/6] KVM: x86/pmu: Refactoring find_arch_event() to pmc_perf_hw

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Message-ID: <CALMp9eRaZBftkaFsmfH8V519QdSGKTORp0OAZ2WaNi3f9X=tng@mail.gmail.com>
Date:   Wed, 8 Dec 2021 19:52:20 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Like Xu <likexu@...cent.com>
Subject: Re: [PATCH v2 2/6] KVM: x86/pmu: Refactoring find_arch_event() to pmc_perf_hw_id()

On Mon, Nov 29, 2021 at 11:42 PM Like Xu <like.xu.linux@...il.com> wrote:
>
> From: Like Xu <likexu@...cent.com>
>
> The find_arch_event() returns a "unsigned int" value,
> which is used by the pmc_reprogram_counter() to
> program a PERF_TYPE_HARDWARE type perf_event.
>
> The returned value is actually the kernel defined generic

Typo: generic.

> perf_hw_id, let's rename it to pmc_perf_hw_id() with simpler
> incoming parameters for better self-explanation.
>
> Signed-off-by: Like Xu <likexu@...cent.com>
> ---
>  arch/x86/kvm/pmu.c           | 8 +-------
>  arch/x86/kvm/pmu.h           | 3 +--
>  arch/x86/kvm/svm/pmu.c       | 8 ++++----
>  arch/x86/kvm/vmx/pmu_intel.c | 9 +++++----
>  4 files changed, 11 insertions(+), 17 deletions(-)
>
> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index 09873f6488f7..3b3ccf5b1106 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -174,7 +174,6 @@ static bool pmc_resume_counter(struct kvm_pmc *pmc)
>  void reprogram_gp_counter(struct kvm_pmc *pmc, u64 eventsel)
>  {
>         unsigned config, type = PERF_TYPE_RAW;
> -       u8 event_select, unit_mask;
>         struct kvm *kvm = pmc->vcpu->kvm;
>         struct kvm_pmu_event_filter *filter;
>         int i;
> @@ -206,17 +205,12 @@ void reprogram_gp_counter(struct kvm_pmc *pmc, u64 eventsel)
>         if (!allow_event)
>                 return;
>
> -       event_select = eventsel & ARCH_PERFMON_EVENTSEL_EVENT;
> -       unit_mask = (eventsel & ARCH_PERFMON_EVENTSEL_UMASK) >> 8;
> -
>         if (!(eventsel & (ARCH_PERFMON_EVENTSEL_EDGE |
>                           ARCH_PERFMON_EVENTSEL_INV |
>                           ARCH_PERFMON_EVENTSEL_CMASK |
>                           HSW_IN_TX |
>                           HSW_IN_TX_CHECKPOINTED))) {

The mechanics of the change look fine, but I do have some questions,
for my own understanding.

Why don't we just use PERF_TYPE_RAW for guest counters all of the
time? What is the advantage of matching entries in a table so that we
can use PERF_TYPE_HARDWARE?

Why do the HSW_IN_TX* bits result in bypassing this clause, when these
bits are extracted as arguments to pmc_reprogram_counter below?

Reviewed-by: Jim Mattson <jmattson@...gle.com>