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

[Alexander Shishkin <alexander.shishkin@linux.intel.com>]

Peter Zijlstra <peterz@infradead.org> writes:

> On Mon, Oct 13, 2014 at 04:45:40PM +0300, Alexander Shishkin wrote:
>> +
>> +enum cpuid_regs {
>> +	CR_EAX = 0,
>> +	CR_ECX,
>> +	CR_EDX,
>> +	CR_EBX
>> +};
>> +
>> +/*
>> + * Capabilities of Intel PT hardware, such as number of address bits or
>> + * supported output schemes, are cached and exported to userspace as "caps"
>> + * attribute group of pt pmu device
>> + * (/sys/bus/event_source/devices/intel_pt/caps/) so that userspace can store
>> + * relevant bits together with intel_pt traces.
>> + */
>> +#define PT_CAP(_n, _l, _r, _m)						\
>> +	[PT_CAP_ ## _n] = { .name = __stringify(_n), .leaf = _l,	\
>> +			    .reg = _r, .mask = _m }
>> +
>> +static struct pt_cap_desc {
>> +	const char	*name;
>> +	u32		leaf;
>> +	u8		reg;
>> +	u32		mask;
>> +} pt_caps[] = {
>> +	PT_CAP(max_subleaf,		0, CR_EAX, 0xffffffff),
>> +	PT_CAP(cr3_filtering,		0, CR_EBX, BIT(0)),
>> +	PT_CAP(topa_output,		0, CR_ECX, BIT(0)),
>> +	PT_CAP(topa_multiple_entries,	0, CR_ECX, BIT(1)),
>> +	PT_CAP(payloads_lip,		0, CR_ECX, BIT(31)),
>> +};
>> +
>> +static u32 pt_cap_get(enum pt_capabilities cap)
>> +{
>> +	struct pt_cap_desc *cd = &pt_caps[cap];
>> +	u32 c = pt_pmu.caps[cd->leaf * 4 + cd->reg];
>> +	unsigned int shift = __ffs(cd->mask);
>> +
>> +	return (c & cd->mask) >> shift;
>> +}
>
>> +	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?

Regards,
--
Alex