The upcoming follow-fork-mode child support requires knowing when
fork() is about to happen in order to initialize its state. Add a hook
for that.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 bsd-user/main.c        | 1 +
 gdbstub/user.c         | 4 ++++
 include/gdbstub/user.h | 5 +++++
 linux-user/main.c      | 1 +
 4 files changed, 11 insertions(+)

diff --git a/bsd-user/main.c b/bsd-user/main.c
index 4140edc8311..bfe6888ea89 100644
--- a/bsd-user/main.c
+++ b/bsd-user/main.c
@@ -106,6 +106,7 @@ void fork_start(void)
     start_exclusive();
     cpu_list_lock();
     mmap_fork_start();
+    gdbserver_fork_start();
 }
 
 void fork_end(int child)
diff --git a/gdbstub/user.c b/gdbstub/user.c
index e6809da2243..85fd44b6aa9 100644
--- a/gdbstub/user.c
+++ b/gdbstub/user.c
@@ -356,6 +356,10 @@ int gdbserver_start(const char *port_or_path)
     return -1;
 }
 
+void gdbserver_fork_start(void)
+{
+}
+
 static void disable_gdbstub(void)
 {
     CPUState *cpu;
diff --git a/include/gdbstub/user.h b/include/gdbstub/user.h
index 68b6534130c..e33f8d9a9a6 100644
--- a/include/gdbstub/user.h
+++ b/include/gdbstub/user.h
@@ -45,6 +45,11 @@ static inline int gdb_handlesig(CPUState *cpu, int sig)
  */
 void gdb_signalled(CPUArchState *as, int sig);
 
+/**
+ * gdbserver_fork_start() - inform gdb of the upcoming fork()
+ */
+void gdbserver_fork_start(void);
+
 /**
  * gdbserver_fork() - disable gdb stub for child processes.
  * @cs: CPU
diff --git a/linux-user/main.c b/linux-user/main.c
index e6427d72332..8c7bea1c631 100644
--- a/linux-user/main.c
+++ b/linux-user/main.c
@@ -144,6 +144,7 @@ void fork_start(void)
     mmap_fork_start();
     cpu_list_lock();
     qemu_plugin_user_prefork_lock();
+    gdbserver_fork_start();
 }
 
 void fork_end(int child)
-- 
2.43.0