linux-kernel - Re: [PATCH v5 12/20] x86: perf: intel

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Message-ID: <20141104155734.GL12706@worktop.programming.kicks-ass.net>
Date:	Tue, 4 Nov 2014 16:57:34 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Robert Richter <rric@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>,
	Andi Kleen <ak@...ux.intel.com>, kan.liang@...el.com,
	adrian.hunter@...el.com, acme@...radead.org
Subject: Re: [PATCH v5 12/20] x86: perf: intel_pt: Intel PT PMU driver

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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