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

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Message-ID: <ebfac3c7-fbc6-78a5-50c5-005ea11cc6ca@gmail.com>
Date:   Thu, 9 Dec 2021 16:11:50 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Jim Mattson <jmattson@...gle.com>,
        "Peter Zijlstra (Intel OTC, Netherlander)" <peterz@...radead.org>
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 9/12/2021 11:52 am, Jim Mattson wrote:
> 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?

The first reason is we need PERF_TYPE_HARDWARE for fixed counters.

And then we might wonder whether we can create perf-event faster
using PERF_TYPE_HARDWARE compared to PERF_TYPE_HARDWARE.

But the (current) answer is no, and probably the opposite:

# The cost (nanosecond) of calling perf_event_create_kernel_counter()
PERF_TYPE_RAW
Max= 1072211
Min= 11122
Avg= 41681.7

PERF_TYPE_HARDWARE
Max= 46184215
Min= 16194
Avg= 250650

So why don't we just use PERF_TYPE_RAW for just all gp counters ?

Hi Peter, do you have any comments to invalidate this proposal ?

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

Once upon the time, the "PERF_TYPE_RAW" was introduced in the
perf with comment "available TYPE space, raw is the max value",
which means, per my understanding, it's our final type choice
for creating a valid perf_event when HSW_IN_TX* bits are set
and KVM needs to hack other perf_event_attr stuff for this
HSW_IN_TX feature with the help of extracted arguments.

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