gdbstub: add multiprocess support to '?' packets

The gdb_get_cpu_pid() function does the PID lookup for the given CPU. It
checks if the CPU is a direct child of a CPU cluster. If it is, the
returned PID is the cluster ID plus one (cluster IDs start at 0, GDB
PIDs at 1). When the CPU is not a child of such a container, the PID of
the default process is returned.

The gdb_fmt_thread_id() function generates the string to be used to identify
a given thread, in a response packet for the peer. This function
supports generating thread IDs when multiprocess mode is enabled (in the
form `p<pid>.<tid>').

Use them in the reply to a '?' request.

Signed-off-by: Luc Michel <luc.michel@greensocs.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 20181207090135.7651-4-luc.michel@greensocs.com
[PMM: fixed checkpatch blockquote style nit]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

gdbstub.c

@@ -640,6 +640,54 @@ static int memtox(char *buf, const char *mem, int len)
     return p - buf;
 }
 
+static uint32_t gdb_get_cpu_pid(const GDBState *s, CPUState *cpu)
+{
+#ifndef CONFIG_USER_ONLY
+    gchar *path, *name = NULL;
+    Object *obj;
+    CPUClusterState *cluster;
+    uint32_t ret;
+
+    path = object_get_canonical_path(OBJECT(cpu));
+
+    if (path == NULL) {
+        /* Return the default process' PID */
+        ret = s->processes[s->process_num - 1].pid;
+        goto out;
+    }
+
+    name = object_get_canonical_path_component(OBJECT(cpu));
+    assert(name != NULL);
+
+    /*
+     * Retrieve the CPU parent path by removing the last '/' and the CPU name
+     * from the CPU canonical path.
+     */
+    path[strlen(path) - strlen(name) - 1] = '\0';
+
+    obj = object_resolve_path_type(path, TYPE_CPU_CLUSTER, NULL);
+
+    if (obj == NULL) {
+        /* Return the default process' PID */
+        ret = s->processes[s->process_num - 1].pid;
+        goto out;
+    }
+
+    cluster = CPU_CLUSTER(obj);
+    ret = cluster->cluster_id + 1;
+
+out:
+    g_free(name);
+    g_free(path);
+
+    return ret;
+
+#else
+    /* TODO: In user mode, we should use the task state PID */
+    return s->processes[s->process_num - 1].pid;
+#endif
+}
+
 static const char *get_feature_xml(const char *p, const char **newp,
                                    CPUClass *cc)
 {
@@ -909,6 +957,19 @@ static CPUState *find_cpu(uint32_t thread_id)
     return NULL;
 }
 
+static char *gdb_fmt_thread_id(const GDBState *s, CPUState *cpu,
+                           char *buf, size_t buf_size)
+{
+    if (s->multiprocess) {
+        snprintf(buf, buf_size, "p%02x.%02x",
+                 gdb_get_cpu_pid(s, cpu), cpu_gdb_index(cpu));
+    } else {
+        snprintf(buf, buf_size, "%02x", cpu_gdb_index(cpu));
+    }
+
+    return buf;
+}
+
 static int is_query_packet(const char *p, const char *query, char separator)
 {
     unsigned int query_len = strlen(query);
@@ -1020,6 +1081,7 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf)
     int ch, reg_size, type, res;
     uint8_t mem_buf[MAX_PACKET_LENGTH];
     char buf[sizeof(mem_buf) + 1 /* trailing NUL */];
+    char thread_id[16];
     uint8_t *registers;
     target_ulong addr, len;
 
@@ -1030,8 +1092,8 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf)
     switch(ch) {
     case '?':
         /* TODO: Make this return the correct value for user-mode.  */
-        snprintf(buf, sizeof(buf), "T%02xthread:%02x;", GDB_SIGNAL_TRAP,
-                 cpu_gdb_index(s->c_cpu));
+        snprintf(buf, sizeof(buf), "T%02xthread:%s;", GDB_SIGNAL_TRAP,
+                 gdb_fmt_thread_id(s, s->c_cpu, thread_id, sizeof(thread_id)));
         put_packet(s, buf);
         /* Remove all the breakpoints when this query is issued,
          * because gdb is doing and initial connect and the state