Re: [PATCH v5 12/20] x86: perf: intel_pt: Intel PT PMU driver

[Peter Zijlstra]

On Fri, Oct 31, 2014 at 03:13:35PM +0200, Alexander Shishkin wrote:
> >> +	if (test_cpu_cap(&boot_cpu_data, X86_FEATURE_INTEL_PT)) {
> >> +		for (i = 0; i < PT_CPUID_LEAVES; i++)
> >> +			cpuid_count(20, i,
> >> +				    &pt_pmu.caps[CR_EAX + i * 4],
> >> +				    &pt_pmu.caps[CR_EBX + i * 4],
> >> +				    &pt_pmu.caps[CR_ECX + i * 4],
> >> +				    &pt_pmu.caps[CR_EDX + i * 4]);
> >> +	} else
> >> +		return -ENODEV;
> >
> > I would really rather you use bitfield unions for cpuid stuff, have a
> > look at union cpuid10_e[abd]x as used in
> > perf_event_intel.c:intel_pmu_init().
> 
> It looks like it would only work for the first cpuid leaf, but we'll
> need more than that. And the array makes it easier to allocate
> attributes for sysfs en masse.
> 
> I guess it doesn't really matter if we use unions unless these bits need
> to be exported to other parts of the kernel? But *that* is hardly a good
> idea. What do you think?

Ah yes, the generation. C is lacking there isn't it :/

Now I'm not sure we want to export all the bits you're using though.
Like the topa_multiple_entires, that appears an implementation detail
userspace should not really care about either way.


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