Re: [PATCH PATCH 3/9] perf/x86/intel: Use topology_cpu_type() to get cpu-type

[Pawan Gupta]

On Mon, Jun 17, 2024 at 07:50:53AM -0700, Dave Hansen wrote:
> On 6/17/24 02:11, Pawan Gupta wrote:
> > find_hybrid_pmu_for_cpu() uses get_this_hybrid_cpu_type() to get the CPU
> > type, but it returns an invalid cpu-type when X86_FEATURE_HYBRID_CPU is not
> > set. Some hybrid variants do enumerate cpu-type regardless of
> > X86_FEATURE_HYBRID_CPU.
> 
> I'm not fully sure what point this is trying to make.

Sorry it was not clear. I will rephrase it.

> Is this trying to make the case that get_this_hybrid_cpu_type() and
> topology_cpu_type() are equivalent or pointing out a difference?

Pointing out a difference. get_this_hybrid_cpu_type() misses a case when
cpu-type is enumerated regardless of X86_FEATURE_HYBRID_CPU. I don't think
checking for the hybrid feature is necessary here, because there is an
existing fixup for this case:

  static struct x86_hybrid_pmu *find_hybrid_pmu_for_cpu(void)
  {
          u8 cpu_type = topology_cpu_type(smp_processor_id());
          int i;

          /*
           * This is running on a CPU model that is known to have hybrid
           * configurations. But the CPU told us it is not hybrid, shame
           * on it. There should be a fixup function provided for these
           * troublesome CPUs (->get_hybrid_cpu_type).
           */
          if (cpu_type == HYBRID_INTEL_NONE) {
                  if (x86_pmu.get_hybrid_cpu_type)
                          cpu_type = x86_pmu.get_hybrid_cpu_type();
                  else
                          return NULL;
          }