perf/x86: Add MSR probe interface

Adding perf_msr_probe function to provide interface for
checking up on MSR register and set the related attribute
group visibility.

User defines following struct for each MSR register:

  struct perf_msr {
       u64                       msr;
       struct attribute_group   *grp;
       bool                    (*test)(int idx, void *data);
       bool                      no_check;
  };

Where:
  msr      - is the MSR address
  attrs    - is attribute groups array to add if the check passed
  test     - is test function pointer
  no_check - is bool that bypass the check and adds the
              attribute without any test

The array of struct perf_msr is passed into:

  perf_msr_probe(struct perf_msr *msr, int cnt, bool zero, void *data)

Together with:
  cnt  - which is the number of struct msr array elements
  data - which is user pointer passed to the test function
  zero - allow counters that returns zero on rdmsr

The perf_msr_probe will executed test code, read the MSR and
check the value is != 0. If all these tests pass, related
attribute group is kept visible.

Also adding PMU_EVENT_GROUP macro helper to define attribute
group for single attribute. It will be used in following patches.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan <kan.liang@linux.intel.com>
Cc: Liang
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Link: https://lkml.kernel.org/r/20190616140358.27799-2-jolsa@kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/events/Makefile

 # SPDX-License-Identifier: GPL-2.0-only
-obj-y					+= core.o
+obj-y					+= core.o probe.o
 obj-y					+= amd/
 obj-$(CONFIG_X86_LOCAL_APIC)            += msr.o
 obj-$(CONFIG_CPU_SUP_INTEL)		+= intel/

arch/x86/events/probe.c

+// SPDX-License-Identifier: GPL-2.0
+#include <linux/export.h>
+#include <linux/types.h>
+#include <linux/bits.h>
+#include "probe.h"
+
+static umode_t
+not_visible(struct kobject *kobj, struct attribute *attr, int i)
+{
+	return 0;
+}
+
+unsigned long
+perf_msr_probe(struct perf_msr *msr, int cnt, bool zero, void *data)
+{
+	unsigned long avail = 0;
+	unsigned int bit;
+	u64 val;
+
+	if (cnt >= BITS_PER_LONG)
+		return 0;
+
+	for (bit = 0; bit < cnt; bit++) {
+		if (!msr[bit].no_check) {
+			struct attribute_group *grp = msr[bit].grp;
+
+			grp->is_visible = not_visible;
+
+			if (msr[bit].test && !msr[bit].test(bit, data))
+				continue;
+			/* Virt sucks; you cannot tell if a R/O MSR is present :/ */
+			if (rdmsrl_safe(msr[bit].msr, &val))
+				continue;
+			/* Disable zero counters if requested. */
+			if (!zero && !val)
+				continue;
+
+			grp->is_visible = NULL;
+		}
+		avail |= BIT(bit);
+	}
+
+	return avail;
+}
+EXPORT_SYMBOL_GPL(perf_msr_probe);

arch/x86/events/probe.h

+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ARCH_X86_EVENTS_PROBE_H__
+#define __ARCH_X86_EVENTS_PROBE_H__
+#include <linux/sysfs.h>
+
+struct perf_msr {
+	u64			  msr;
+	struct attribute_group	 *grp;
+	bool			(*test)(int idx, void *data);
+	bool			  no_check;
+};
+
+unsigned long
+perf_msr_probe(struct perf_msr *msr, int cnt, bool no_zero, void *data);
+
+#define __PMU_EVENT_GROUP(_name)			\
+static struct attribute *attrs_##_name[] = {		\
+	&attr_##_name.attr.attr,			\
+	NULL,						\
+}
+
+#define PMU_EVENT_GROUP(_grp, _name)			\
+__PMU_EVENT_GROUP(_name);				\
+static struct attribute_group group_##_name = {		\
+	.name  = #_grp,					\
+	.attrs = attrs_##_name,				\
+}
+
+#endif /* __ARCH_X86_EVENTS_PROBE_H__ */