[PATCH V2] vl: pause option

[Steve Sistare <steven.sistare@oracle.com>]

Provide the -pause command-line parameter and the QEMU_PAUSE environment
variable to pause QEMU during process startup using SIGSTOP and allow a
developer to attach a debugger, or observe the process using tools such as
strace.  Useful when the QEMU has been launched with some other entity, such
as libvirt.  QEMU_PAUSE is checked in a constructor at the highest priority,
and can be used to debug other constructors.  The -pause option is checked
later, during argument processing in main, but is useful if passing an
environment variable from a launcher to qemu is awkard.

Usage: qemu -pause, or QEMU_PAUSE=1
After attaching a debugger, send SIGCONT to the qemu process to continue.

Example:

$ QEMU_PAUSE=1 qemu-system-x86_64 ...
QEMU pid 18371 is stopped.
[1]+  Stopped
                                 $ gdb -p 18371
                                 (gdb)
$ kill -cont 18371
                                 (gdb) break rcu_init
                                 (gdb) continue
                                 Program received signal SIGCONT, Continued.
                                 (gdb) continue
                                 Breakpoint 1, rcu_init () at util/rcu.c:380

Thanks to Daniel P. Berrange <berrange@redhat.com> for suggesting SIGSTOP.

Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
---
 qemu-options.hx | 14 ++++++++++++++
 softmmu/vl.c    | 23 +++++++++++++++++++++++
 2 files changed, 37 insertions(+)

diff --git a/qemu-options.hx b/qemu-options.hx
index 708583b..42edd70 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -3668,6 +3668,20 @@ SRST
     option is experimental.
 ERST
 
+DEF("pause", 0, QEMU_OPTION_pause, \
+    "-pause          pause the qemu process in main using SIGSTOP.\n"
+    "                to pause earlier, before constructors are run, set the\n"
+    "                environment variable QEMU_PAUSE=1 before starting qemu.\n",
+    QEMU_ARCH_ALL)
+
+SRST
+``-pause``
+    Pause the qemu process in main using SIGSTOP.  This is useful for attaching
+    a debugger after QEMU has been launched by some other entity.  After
+    attaching, send SIGCONT to continue.  To pause earlier, before constructors
+    are run, set the environment variable QEMU_PAUSE=1 before starting qemu.
+ERST
+
 DEF("S", 0, QEMU_OPTION_S, \
     "-S              freeze CPU at startup (use 'c' to start execution)\n",
     QEMU_ARCH_ALL)
diff --git a/softmmu/vl.c b/softmmu/vl.c
index 4eb9d1f..aee1a96 100644
--- a/softmmu/vl.c
+++ b/softmmu/vl.c
@@ -2829,6 +2829,24 @@ static void create_default_memdev(MachineState *ms, const char *path)
                             &error_fatal);
 }
 
+static __attribute__((constructor(101))) void maybe_pause(void)
+{
+    const char *pause = getenv("QEMU_PAUSE");
+
+    if (pause) {
+        if (!pause[0] || !strcmp(pause, "1")) {
+            printf("QEMU pid %d is stopped.  Send SIGCONT to continue.\n",
+                   getpid());
+            kill(getpid(), SIGSTOP);
+        } else if (strcmp(pause, "0")) {
+            fprintf(stderr, "error: QEMU_PAUSE bad value %s. Must be 1 or "
+                            "empty to enable, 0 or unset to disable.\n",
+                            pause);
+            exit(1);
+        }
+    }
+}
+
 void qemu_init(int argc, char **argv, char **envp)
 {
     int i;
@@ -3191,6 +3209,11 @@ void qemu_init(int argc, char **argv, char **envp)
             case QEMU_OPTION_gdb:
                 add_device_config(DEV_GDB, optarg);
                 break;
+            case QEMU_OPTION_pause:
+                printf("QEMU pid %d is stopped.  Send SIGCONT to continue.\n",
+                       getpid());
+                kill(getpid(), SIGSTOP);
+                break;
             case QEMU_OPTION_L:
                 if (is_help_option(optarg)) {
                     list_data_dirs = true;
-- 
1.8.3.1