linux-kernel - Re: [PATCH v4 04/10] KVM/x86: intel_pmu_lbr

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Date:   Tue, 08 Jan 2019 14:13:16 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        pbonzini@...hat.com, ak@...ux.intel.com, peterz@...radead.org
CC:     kan.liang@...el.com, mingo@...hat.com, rkrcmar@...hat.com,
        like.xu@...el.com, jannh@...gle.com, arei.gonglei@...wei.com
Subject: Re: [PATCH v4 04/10] KVM/x86: intel_pmu_lbr_enable

On 01/07/2019 10:22 PM, Liang, Kan wrote:
>
>> Thanks for sharing. I understand the point of maintaining those 
>> models at one place,
>> but this factor-out doesn't seem very elegant to me, like below
>>
>> __intel_pmu_init (int model, struct x86_pmu *x86_pmu)
>> {
>> ...
>> switch (model)
>> case INTEL_FAM6_NEHALEM:
>> case INTEL_FAM6_NEHALEM_EP:
>> case INTEL_FAM6_NEHALEM_EX:
>>      intel_pmu_lbr_init(x86_pmu);
>>      if (model != boot_cpu_data.x86_model)
>>          return;
>>
>>      /* Other a lot of things init like below..*/
>>      memcpy(hw_cache_event_ids, nehalem_hw_cache_event_ids,
>>                     sizeof(hw_cache_event_ids));
>>      memcpy(hw_cache_extra_regs, nehalem_hw_cache_extra_regs,
>>                     sizeof(hw_cache_extra_regs));
>>      x86_pmu.event_constraints = intel_nehalem_event_constraints;
>>                  x86_pmu.pebs_constraints = 
>> intel_nehalem_pebs_event_constraints;
>>                  x86_pmu.enable_all = intel_pmu_nhm_enable_all;
>>                  x86_pmu.extra_regs = intel_nehalem_extra_regs;
>>   ...
>>
>> Case...
>> }
>> We need insert "if (model != boot_cpu_data.x86_model)" in every "Case 
>> xx".
>>
>> What would be the rationale that we only do lbr_init for "x86_pmu"
>> when model != boot_cpu_data.x86_model?
>> (It looks more like a workaround to factor-out the function and get 
>> what we want)
>
> I thought the new function may be extended to support fake pmu as below.
> It's not only for lbr. PMU has many CPU specific features. It can be 
> used for other features, if you want to check the compatibility in 
> future. But I don't have an example now.
>
> __intel_pmu_init (int model, struct x86_pmu *x86_pmu)
> {
> bool fake_pmu = (model != boot_cpu_data.x86_model) ? true : false;
> ...
> switch (model)
> case INTEL_FAM6_NEHALEM:
> case INTEL_FAM6_NEHALEM_EP:
> case INTEL_FAM6_NEHALEM_EX:
>      intel_pmu_lbr_init(x86_pmu);
>      x86_pmu->event_constraints = intel_nehalem_event_constraints;
>      x86_pmu->pebs_constraints = intel_nehalem_pebs_event_constraints;
>      x86_pmu->enable_all = intel_pmu_nhm_enable_all;
>      x86_pmu->extra_regs = intel_nehalem_extra_regs;
>
>      if (fake_pmu)
>          return;

It looks similar as the one I shared above, the difference is that more 
things
(e.g. constraints) are assigned to x86_fake_pmu.
I'm not sure about the logic behind it (still look like a workaround).



>
>      /* Global variables should not be updated for fake PMU */
>      memcpy(hw_cache_event_ids, nehalem_hw_cache_event_ids,
>                     sizeof(hw_cache_event_ids));
>      memcpy(hw_cache_extra_regs, nehalem_hw_cache_extra_regs,
>                     sizeof(hw_cache_extra_regs));
>
>
>>
>> I would prefer having them separated as this patch for now - it is 
>> logically more clear to me.
>>
>
> But it will be a problem for maintenance. Perf developer probably 
> forget to update the list in KVM. I think you have to regularly check 
> the perf code.
>

It's been very common in hypervisor development. That's why we have 
hypervisor developers here.
When a new platform is added, we will definitely get some work like this 
to do.

Best,
Wei