Re: [PATCH v4 04/10] KVM/x86: intel_pmu_lbr_enable

[Wei Wang]

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