[PATCH v1 3/5] perf tools arm64: Add support for get_cpuid() function

[Zenghui Yu <yuzenghui@huawei.com>]

The get_cpuid() function returns the MIDR string of the first CPU.

Cc: Ganapatrao Kulkarni <ganapatrao.kulkarni@cavium.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>

---
When recording, perf will write host's cpuid (through get_cpuid()) into
perf.data.guest file:

  __cmd_record
    record__finish_output
      perf_session__write_header
        perf_header__adds_write
          do_write_feat (type == HEADER_CPUID)
            write_cpuid

When reporting, perf will read cpuid from perf.data.guest file:

  read_events
    perf_session__new
      perf_session__open
        perf_session__read_header
          perf_header__process_sections
            perf_file_section__process
              process_cpuid

I'm not familiar with ARM ID register usage, and how does perf code make
use of get_cpuid() function. If we left get_cpuid() unimplemented on
arm64 (we have a default implementation in tools/perf/util/header.c),
'perf kvm stat report' will failed with following error:
        "Failed to look up CPU type"

I will read the code further, and any comments or suggestions from both
sides (arm64 & perf) will be appreciated.

---
 tools/perf/arch/arm64/util/header.c | 74 +++++++++++++++++++++++--------------
 1 file changed, 47 insertions(+), 27 deletions(-)

diff --git a/tools/perf/arch/arm64/util/header.c b/tools/perf/arch/arm64/util/header.c
index 534cd25..5c17b86 100644
--- a/tools/perf/arch/arm64/util/header.c
+++ b/tools/perf/arch/arm64/util/header.c
@@ -9,17 +9,56 @@
 #define MIDR_VARIANT_SHIFT      20
 #define MIDR_VARIANT_MASK       (0xf << MIDR_VARIANT_SHIFT)
 
-char *get_cpuid_str(struct perf_pmu *pmu)
+/* Return value of midr_el1 if success, else return 0. */
+static int read_midr_el1(char *buf, int cpu_nr)
 {
-	char *buf = NULL;
 	char path[PATH_MAX];
 	const char *sysfs = sysfs__mountpoint();
+	u64 midr = 0;
+	FILE *file;
+
+	if (!sysfs)
+		return 0;
+
+	scnprintf(path, PATH_MAX, "%s/devices/system/cpu/cpu%d"MIDR,
+		  sysfs, cpu_nr);
+
+	file = fopen(path, "r");
+	if (!file) {
+		pr_debug("fopen failed for file %s\n", path);
+		return 0;
+	}
+
+	if (!fgets(buf, MIDR_SIZE, file)) {
+		fclose(file);
+		return 0;
+	}
+	fclose(file);
+
+	/* Ignore/clear Variant[23:20] and Revision[3:0] of MIDR */
+	midr = strtoul(buf, NULL, 16);
+	midr &= (~(MIDR_VARIANT_MASK | MIDR_REVISION_MASK));
+	scnprintf(buf, MIDR_SIZE, "0x%016lx", midr);
+
+	return midr;
+}
+
+int get_cpuid(char *buffer, size_t sz __maybe_unused)
+{
+	if (read_midr_el1(buffer, 0))
+		return 0;
+
+	return -1;
+}
+
+char *get_cpuid_str(struct perf_pmu *pmu)
+{
+	char *buf = NULL;
 	int cpu;
 	u64 midr = 0;
 	struct cpu_map *cpus;
-	FILE *file;
 
-	if (!sysfs || !pmu || !pmu->cpus)
+	if (!pmu || !pmu->cpus)
 		return NULL;
 
 	buf = malloc(MIDR_SIZE);
@@ -29,29 +68,10 @@ char *get_cpuid_str(struct perf_pmu *pmu)
 	/* read midr from list of cpus mapped to this pmu */
 	cpus = cpu_map__get(pmu->cpus);
 	for (cpu = 0; cpu < cpus->nr; cpu++) {
-		scnprintf(path, PATH_MAX, "%s/devices/system/cpu/cpu%d"MIDR,
-				sysfs, cpus->map[cpu]);
-
-		file = fopen(path, "r");
-		if (!file) {
-			pr_debug("fopen failed for file %s\n", path);
-			continue;
-		}
-
-		if (!fgets(buf, MIDR_SIZE, file)) {
-			fclose(file);
-			continue;
-		}
-		fclose(file);
-
-		/* Ignore/clear Variant[23:20] and
-		 * Revision[3:0] of MIDR
-		 */
-		midr = strtoul(buf, NULL, 16);
-		midr &= (~(MIDR_VARIANT_MASK | MIDR_REVISION_MASK));
-		scnprintf(buf, MIDR_SIZE, "0x%016lx", midr);
-		/* got midr break loop */
-		break;
+		midr = read_midr_el1(buf, cpus->map[cpu]);
+		if (midr)
+			/* got midr break loop */
+			break;
 	}
 
 	if (!midr) {
-- 
1.8.3.1