perf metrics: Add has_pmem literal

Add literal so that if nvdimms aren't installed we can record fewer
events.  The file detection mechanism was suggested by Dan Williams
<dan.j.williams@intel.com> in:

  https://lore.kernel.org/linux-perf-users/641bbe1eced26_1b98bb29440@dwillia2-xfh.jf.intel.com.notmuch/Reviewed-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Caleb Biggers <caleb.biggers@intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Edward Baker <edward.baker@intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Perry Taylor <perry.taylor@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Samantha Alt <samantha.alt@intel.com>
Cc: Weilin Wang <weilin.wang@intel.com>
Cc: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Link: https://lore.kernel.org/r/20230324072218.181880-2-irogers@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/expr.c

@@ -14,6 +14,7 @@
 #include "util/hashmap.h"
 #include "smt.h"
 #include "tsc.h"
+#include <api/fs/fs.h>
 #include <linux/err.h>
 #include <linux/kernel.h>
 #include <linux/zalloc.h>
@@ -400,6 +401,20 @@ double arch_get_tsc_freq(void)
 }
 #endif
 
+static double has_pmem(void)
+{
+	static bool has_pmem, cached;
+	const char *sysfs = sysfs__mountpoint();
+	char path[PATH_MAX];
+
+	if (!cached) {
+		snprintf(path, sizeof(path), "%s/firmware/acpi/tables/NFIT", sysfs);
+		has_pmem = access(path, F_OK) == 0;
+		cached = true;
+	}
+	return has_pmem ? 1.0 : 0.0;
+}
+
 double expr__get_literal(const char *literal, const struct expr_scanner_ctx *ctx)
 {
 	const struct cpu_topology *topology;
@@ -449,6 +464,10 @@ double expr__get_literal(const char *literal, const struct expr_scanner_ctx *ctx
 		result = perf_pmu__cpu_slots_per_cycle();
 		goto out;
 	}
+	if (!strcmp("#has_pmem", literal)) {
+		result = has_pmem();
+		goto out;
+	}
 
 	pr_err("Unrecognized literal '%s'", literal);
 out: