[PATCH v3 11/11] gdb, gdbserver: add AlwaysNonStop remote protocol extension

[Mohamed Bouhaouel <mohamed.bouhaouel@intel.com>]

From: "Bouhaouel, Mohamed" <mohamed.bouhaouel@intel.com>

Add a new RSP extension allowing remote stub to declare that a target
must always operate in non-stop mode.

When a stub advertises "AlwaysNonStop+" in its qSupported response:

* The stub automatically initializes to non-stop mode
* GDB will operate the stub in non-stop mode (with default settings)
* The stub will reject QNonStop:0 requests
* GDB errors if 'maint set target-non-stop off' is explicitly set
---
 gdb/NEWS            |  8 ++++++++
 gdb/doc/gdb.texinfo | 30 +++++++++++++++++++++++++++++-
 gdb/remote.c        | 27 +++++++++++++++++++++++++++
 gdbserver/server.cc | 30 +++++++++++++++++++++++++++++-
 gdbserver/target.cc |  6 ++++++
 gdbserver/target.h  |  6 ++++++
 6 files changed, 105 insertions(+), 2 deletions(-)

diff --git a/gdb/NEWS b/gdb/NEWS
index 343828a85dd..b578bb28843 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -91,6 +91,14 @@
   record btrace pt event-tracing on) on a FRED-enabled system and from
   Trigger Tracing.
 
+* New remote protocol extension 'AlwaysNonStop' allows gdbserver to
+  indicate that the target must always operate in non-stop mode and
+  cannot switch to all-stop mode.  When a remote stub advertises
+  'AlwaysNonStop+' in its qSupported reply, GDB will operate the stub
+  in non-stop mode (when 'maint set target-non-stop' is 'auto',
+  the default).  GDB will refuse to connect if 'maint set target-non-stop'
+  is explicitly set to 'off'.
+
 * Configure changes
 
 ** --with-babeltrace has been removed.  The babeltrace library was
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index a698b2b8451..dedd5ef3b59 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -42922,7 +42922,8 @@ does not indicate support.
 
 @item maint set target-non-stop off
 @value{GDBN} does not control the target in non-stop mode even if the
-target supports it.
+target supports it.  Note: connection to targets that require non-stop
+mode (via @samp{AlwaysNonStop+}) will fail with this setting.
 @end table
 
 Here is how @code{set non-stop} and @code{maint set target-non-stop}
@@ -45094,6 +45095,11 @@ Reply:
 @table @samp
 @item OK
 The request succeeded.
+
+@item @samp{E @var{nn}}
+The request failed.  This may occur if the stub requires non-stop mode
+and advertised @samp{AlwaysNonStop+} support, and GDB attempted to
+switch to all-stop mode with @samp{QNonStop:0}.
 @end table
 
 This packet is not probed by default; the remote stub must request it,
@@ -45511,6 +45517,21 @@ multiple-watchpoint-addresses-packet} command (@pxref{Remote
 Configuration, set remote multiple-watchpoint-addresses-packet}).
 @end table
 
+@item AlwaysNonStop
+This feature indicates that @value{GDBN} supports remote stubs that
+require non-stop mode operation.  If the stub advertises
+@samp{AlwaysNonStop+}, it will run in non-stop mode, similar to
+the setting @code{maint set target-non-stop on}.  @value{GDBN} will
+not attempt to switch the target to all-stop mode.
+
+Note: @samp{AlwaysNonStop+} requires @samp{QNonStop+} support.
+If a stub advertises @samp{AlwaysNonStop+} without @samp{QNonStop+},
+the @samp{AlwaysNonStop+} extension will be silently ignored by @value{GDBN}.
+
+Use of this feature is controlled by the @code{set remote
+always-non-stop-feature-packet} command (@pxref{Remote
+Configuration, set remote always-non-stop-feature-packet}).
+
 Stubs should ignore any unknown values for
 @var{gdbfeature}.  Any @value{GDBN} which sends a @samp{qSupported}
 packet supports receiving packets of unlimited length (earlier
@@ -45907,6 +45928,13 @@ packet (@pxref{qXfer fdpic loadmap read}).
 The remote stub understands the @samp{QNonStop} packet
 (@pxref{QNonStop}).
 
+@item AlwaysNonStop
+The remote stub requires non-stop mode operation, will reject
+@samp{QNonStop:0} requests, and automatically initializes to non-stop
+mode.  This feature must be advertised together with @samp{QNonStop+};
+if advertised without it, the feature will be silently ignored by
+@value{GDBN}.  @xref{QNonStop}.
+
 @item QCatchSyscalls
 The remote stub understands the @samp{QCatchSyscalls} packet
 (@pxref{QCatchSyscalls}).
diff --git a/gdb/remote.c b/gdb/remote.c
index 7e3537b99df..54a289ace31 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -323,6 +323,9 @@ enum {
   /* Support for the QNonStop packet.  */
   PACKET_QNonStop,
 
+  /* Support for AlwaysNonStop extension.  */
+  PACKET_AlwaysNonStop_feature,
+
   /* Support for the QThreadEvents packet.  */
   PACKET_QThreadEvents,
 
@@ -1442,6 +1445,8 @@ class remote_target : public process_stratum_target
 
   void check_binary_download (CORE_ADDR addr);
 
+  bool always_non_stop_p () override;
+
   target_xfer_status remote_write_bytes_aux (const char *header,
 					     CORE_ADDR memaddr,
 					     const gdb_byte *myaddr,
@@ -5587,6 +5592,12 @@ remote_target::start_remote_1 (int from_tty, int extended_p)
     }
   else if (m_features.packet_support (PACKET_QNonStop) == PACKET_ENABLE)
     {
+      /* If the target requires non-stop mode, don't try to switch to
+	 all-stop mode.  */
+      if (m_features.packet_support (PACKET_AlwaysNonStop_feature)
+	  == PACKET_ENABLE)
+	error (_("Target requires non-stop mode."));
+
       /* Don't assume that the stub can operate in all-stop mode.
 	 Request it explicitly.  */
       putpkt ("QNonStop:0");
@@ -6133,6 +6144,8 @@ static const struct protocol_feature remote_protocol_features[] = {
   { "multiprocess", PACKET_DISABLE, remote_supported_packet,
     PACKET_multiprocess_feature },
   { "QNonStop", PACKET_DISABLE, remote_supported_packet, PACKET_QNonStop },
+  { "AlwaysNonStop", PACKET_DISABLE, remote_supported_packet,
+    PACKET_AlwaysNonStop_feature },
   { "qXfer:siginfo:read", PACKET_DISABLE, remote_supported_packet,
     PACKET_qXfer_siginfo_read },
   { "qXfer:siginfo:write", PACKET_DISABLE, remote_supported_packet,
@@ -9817,6 +9830,18 @@ remote_target::check_binary_download (CORE_ADDR addr)
     }
 }
 
+/* Determine whether the remote target operates in non-stop mode.
+   Returns true if the target advertises AlwaysNonStop, and supports
+   QNonStop for asynchronous execution and stop notifications.  */
+
+bool
+remote_target::always_non_stop_p ()
+{
+  return ((m_features.packet_support (PACKET_QNonStop) == PACKET_ENABLE)
+	  && (m_features.packet_support (PACKET_AlwaysNonStop_feature)
+	      == PACKET_ENABLE));
+}
+
 /* Helper function to resize the payload in order to try to get a good
    alignment.  We try to write an amount of data such that the next write will
    start on an address aligned on REMOTE_ALIGN_WRITES.  */
@@ -17073,6 +17098,8 @@ Show the maximum size of the address (in bits) in a memory packet."), NULL,
 
   add_packet_config_cmd (PACKET_multi_wp_addr,
 			 "multi-wp-addr", "multiple-watchpoint-addresses", 0);
+  add_packet_config_cmd (PACKET_AlwaysNonStop_feature,
+			 "AlwaysNonStop", "always-non-stop-feature", 0);
 
   /* Assert that we've registered "set remote foo-packet" commands
      for all packet configs.  */
diff --git a/gdbserver/server.cc b/gdbserver/server.cc
index baccdf00172..fe8240cbe44 100644
--- a/gdbserver/server.cc
+++ b/gdbserver/server.cc
@@ -838,6 +838,17 @@ handle_general_set (char *own_buf)
 	}
 
       req_str = req ? "non-stop" : "all-stop";
+
+      /* If the target always runs in non-stop mode, reject requests to
+	 switch to all-stop mode.  */
+      if (req == 0 && target_always_non_stop ())
+	{
+	  fprintf (stderr, "Cannot switch to all-stop mode: "
+		   "target requires non-stop mode\n");
+	  write_enn (own_buf);
+	  return;
+	}
+
       if (the_target->start_non_stop (req == 1) != 0)
 	{
 	  fprintf (stderr, "Setting %s mode failed\n", req_str);
@@ -2812,7 +2823,11 @@ handle_query (char *own_buf, int packet_len, int *new_packet_len_p)
 	strcat (own_buf, ";exec-events+");
 
       if (target_supports_non_stop ())
-	strcat (own_buf, ";QNonStop+");
+	{
+	  strcat (own_buf, ";QNonStop+");
+	  if (target_always_non_stop ())
+	    strcat (own_buf, ";AlwaysNonStop+");
+	}
 
       if (target_supports_disable_randomization ())
 	strcat (own_buf, ";QDisableRandomization+");
@@ -2876,6 +2891,19 @@ handle_query (char *own_buf, int packet_len, int *new_packet_len_p)
       if (cs.single_inferior_argument)
 	strcat (own_buf, ";single-inf-arg+");
 
+      /* If the target requires non-stop mode, initialize it now rather
+	 than waiting for GDB to request it via QNonStop:1.  */
+      if (target_supports_non_stop () && target_always_non_stop ())
+	{
+	  if (the_target->start_non_stop (true) != 0)
+	    error ("Cannot initialize target: requires non-stop mode but "
+		   "non-stop mode initialization failed.");
+
+	  non_stop = true;
+	  remote_debug_printf ("Target requires non-stop mode: "
+			       "enabled at startup.\n");
+	}
+
       /* Reinitialize components as needed for the new connection.  */
       hostio_handle_new_gdb_connection ();
       target_handle_new_gdb_connection ();
diff --git a/gdbserver/target.cc b/gdbserver/target.cc
index 43ef421df74..170e40e9002 100644
--- a/gdbserver/target.cc
+++ b/gdbserver/target.cc
@@ -498,6 +498,12 @@ process_stratum_target::supports_non_stop ()
   return false;
 }
 
+bool
+process_stratum_target::always_non_stop ()
+{
+  return false;
+}
+
 bool
 process_stratum_target::async (bool enable)
 {
diff --git a/gdbserver/target.h b/gdbserver/target.h
index 82eab9d1243..cdd1e26f2d6 100644
--- a/gdbserver/target.h
+++ b/gdbserver/target.h
@@ -260,6 +260,9 @@ class process_stratum_target
   /* Return true if non-stop mode is supported.  */
   virtual bool supports_non_stop ();
 
+  /* Return true if target must run in non-stop mode.  */
+  virtual bool always_non_stop ();
+
   /* Enables async target events.  Returns the previous enable
      state.  */
   virtual bool async (bool enable);
@@ -583,6 +586,9 @@ int kill_inferior (process_info *proc);
 #define target_supports_non_stop() \
   the_target->supports_non_stop ()
 
+#define target_always_non_stop() \
+  the_target->always_non_stop ()
+
 #define target_async(enable) \
   the_target->async (enable)
 
-- 
2.43.0

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