perf tools: Automatically use guest kcore_dir if present

When registering a guest machine using machine_pid from the id index,
check perf.data for a matching kcore_dir subdirectory and set the
kallsyms file name accordingly. If set, use it to find the machine's
kernel symbols and object code (from kcore).

Signed-off-by: Adrian Hunter <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Ian Rogers <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/data.c

···
 	return kallsyms_name;
 }
 
+char *perf_data__guest_kallsyms_name(struct perf_data *data, pid_t machine_pid)
+{
+	char *kallsyms_name;
+	struct stat st;
+
+	if (!data->is_dir)
+		return NULL;
+
+	if (asprintf(&kallsyms_name, "%s/kcore_dir__%d/kallsyms", data->path, machine_pid) < 0)
+		return NULL;
+
+	if (stat(kallsyms_name, &st)) {
+		free(kallsyms_name);
+		return NULL;
+	}
+
+	return kallsyms_name;
+}
+
 bool is_perf_data(const char *path)
 {
 	bool ret = false;

tools/perf/util/data.h

···
 int perf_data__make_kcore_dir(struct perf_data *data, char *buf, size_t buf_sz);
 bool has_kcore_dir(const char *path);
 char *perf_data__kallsyms_name(struct perf_data *data);
+char *perf_data__guest_kallsyms_name(struct perf_data *data, pid_t machine_pid);
 bool is_perf_data(const char *path);
 #endif /* __PERF_DATA_H */

tools/perf/util/machine.h

···
 	bool		  single_address_space;
 	char		  *root_dir;
 	char		  *mmap_name;
+	char		  *kallsyms_filename;
 	struct threads    threads[THREADS__TABLE_SIZE];
 	struct vdso_info  *vdso_info;
 	struct perf_env   *env;

tools/perf/util/session.c

···
 		return -ENOMEM;
 	thread__put(thread);
 
+	machine->kallsyms_filename = perf_data__guest_kallsyms_name(session->data, machine_pid);
+
 	return 0;
 }
 

tools/perf/util/symbol.c

···
 static int dso__load_guest_kernel_sym(struct dso *dso, struct map *map)
 {
 	int err;
-	const char *kallsyms_filename = NULL;
+	const char *kallsyms_filename;
 	struct machine *machine = map__kmaps(map)->machine;
 	char path[PATH_MAX];
 
-	if (machine__is_default_guest(machine)) {
+	if (machine->kallsyms_filename) {
+		kallsyms_filename = machine->kallsyms_filename;
+	} else if (machine__is_default_guest(machine)) {
 		/*
 		 * if the user specified a vmlinux filename, use it and only
 		 * it, reporting errors to the user if it cannot be used.