Re: [PATCH v3 5/8] gdbserver: Transmit target description ID in thread list and stop reply

[Andrew Burgess <aburgess@redhat.com>]

Thiago Jung Bauermann via Gdb-patches <gdb-patches@sourceware.org>
writes:

> Now that an inferior thread can have a different target description than
> its process, there needs to be a way to communicate this target
> description to GDB.  So add the concept of a target description ID to the
> remote protocol, which is used to reference them and allows them to be
> transferred only once over the wire.
>
> The ID is an unsigned integer, and is sent in the 'T AA n1:r1;n2:r2;...'
> stop reply packet as a new 'n:r' pair, where n is "tdesc" and "r" is an
> unsigned integer containing the ID.
>
> It is also sent in the threads list XML in the response of a
> qXfer:threads:read request.  The ID is sent as a new "tdesc" attribute of
> the <thread> node.
>
> To request the target description XML of a given ID, GDB sends the
> qXfer:features:read request with "target-id-%u.xml" as the annex, where %u
> is the target description ID.
>
> Suggested-By: Simon Marchi <simon.marchi@efficios.com>
> ---
>  gdb/doc/gdb.texinfo       | 27 ++++++++++++++++---
>  gdbserver/remote-utils.cc | 57 +++++++++++++++++++++++++++++++++++++++
>  gdbserver/remote-utils.h  |  9 +++++++
>  gdbserver/server.cc       | 17 ++++++++++--
>  gdbserver/server.h        |  4 +++
>  5 files changed, 109 insertions(+), 5 deletions(-)
>
> diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
> index 9c0018ea5c14..fbf7e59853b5 100644
> --- a/gdb/doc/gdb.texinfo
> +++ b/gdb/doc/gdb.texinfo
> @@ -42030,6 +42030,10 @@ the stopped thread, as specified in @ref{thread-id syntax}.
>  If @var{n} is @samp{core}, then @var{r} is the hexadecimal number of
>  the core on which the stop event was detected.
>  
> +@item
> +If @var{n} is @samp{tdesc}, then @var{r} is the hexadecimal number of
> +the target description ID (@pxref{Target Description ID}).
> +
>  @item
>  If @var{n} is a recognized @dfn{stop reason}, it describes a more
>  specific event that stopped the target.  The currently defined stop
> @@ -46546,7 +46550,7 @@ the following structure:
>  @smallexample
>  <?xml version="1.0"?>
>  <threads>
> -    <thread id="id" core="0" name="name">
> +    <thread id="id" core="0" name="name" tdesc="0">
>      ... description ...
>      </thread>
>  </threads>
> @@ -46556,8 +46560,10 @@ Each @samp{thread} element must have the @samp{id} attribute that
>  identifies the thread (@pxref{thread-id syntax}).  The
>  @samp{core} attribute, if present, specifies which processor core
>  the thread was last executing on.  The @samp{name} attribute, if
> -present, specifies the human-readable name of the thread.  The content
> -of the of @samp{thread} element is interpreted as human-readable
> +present, specifies the human-readable name of the thread.  The
> +@samp{tdesc} attribute, if present, specifies the target description
> +ID of the thread (@pxref{Target Description ID}).  The content of
> +the @samp{thread} element is interpreted as human-readable
>  auxiliary information.  The @samp{handle} attribute, if present,
>  is a hex encoded representation of the thread handle.
>  
> @@ -46767,6 +46773,8 @@ descriptions are accurate, and that @value{GDBN} understands them.
>  target descriptions.  @xref{Expat}.
>  
>  @menu
> +* Target Description ID::           Referencing different descriptions in the
> +                                    remote protocol.
>  * Retrieving Descriptions::         How descriptions are fetched from a target.
>  * Target Description Format::       The contents of a target description.
>  * Predefined Target Types::         Standard types available for target
> @@ -46775,6 +46783,14 @@ target descriptions.  @xref{Expat}.
>  * Standard Target Features::        Features @value{GDBN} knows about.
>  @end menu
>  
> +@node Target Description ID
> +@section Target Description ID
> +
> +In cases where a remote target supports threads having different
> +target descriptions than their parent process, the remote protocol
> +assigns a non-negative integer to each target description to reference
> +it in the communication between the host and the target.

Having read some of the later patches, I have some additional thoughts
here:

I think we should make it explicit here that IDs are connection wide,
not per-process.  We should also make it clear that GDB might[1] cache
target descriptions per remote connection, and so a remote target should
not reuse a target description ID except where the target description is
identical.

[1] I say "GDB might" here because if we say "GDB will" then this would
imply each target description will only be asked for once.  And I
figure, why be overly restrictive.


Thanks,
Andrew


> +
>  @node Retrieving Descriptions
>  @section Retrieving Descriptions
>  
> @@ -46787,6 +46803,11 @@ qXfer}).  The @var{annex} in the @samp{qXfer} packet will be
>  XML document, of the form described in @ref{Target Description
>  Format}.
>  
> +If target description IDs are being used (@pxref{Target Description ID}),
> +@value{GDBN} can retrieve a target description with a given ID by using
> +@samp{target-id-ID.xml} as the @var{annex}, where @var{ID} is the
> +non-negative integer identifier of the desired target description.
> +
>  Alternatively, you can specify a file to read for the target description.
>  If a file is set, the target will not be queried.  The commands to
>  specify a file are:
> diff --git a/gdbserver/remote-utils.cc b/gdbserver/remote-utils.cc
> index 80310bc2c709..baff899307cc 100644
> --- a/gdbserver/remote-utils.cc
> +++ b/gdbserver/remote-utils.cc
> @@ -1049,6 +1049,53 @@ outreg (struct regcache *regcache, int regno, char *buf)
>    return buf;
>  }
>  
> +/* See remote-utils.h.  */
> +
> +unsigned int
> +get_tdesc_rsp_id (const target_desc *tdesc)
> +{
> +  client_state &cs = get_client_state ();
> +  unsigned int i;
> +
> +  for (i = 0; i < cs.tdescs.size (); i++)
> +    if (cs.tdescs[i] == tdesc)
> +      return i;
> +
> +  cs.tdescs.push_back (tdesc);
> +
> +  return i;
> +}
> +
> +/* See remote-utils.h.  */
> +
> +const target_desc *
> +get_tdesc_from_rsp_id (unsigned int id)
> +{
> +  client_state &cs = get_client_state ();
> +
> +  if (id >= cs.tdescs.size ())
> +    return nullptr;
> +
> +  return cs.tdescs[id];
> +}
> +
> +/* Return the ID as used in the remote protocol for the target descriptor of the
> +   given PTID.  */
> +
> +static unsigned int
> +get_tdesc_rsp_id (ptid_t ptid)
> +{
> +  const thread_info *thread = find_thread_ptid (ptid);
> +  const target_desc *tdesc;
> +
> +  if (thread == nullptr)
> +    tdesc = find_process_pid (ptid.pid ())->tdesc;
> +  else
> +    tdesc = get_thread_target_desc (thread);
> +
> +  return get_tdesc_rsp_id (tdesc);
> +}
> +
>  void
>  prepare_resume_reply (char *buf, ptid_t ptid, const target_waitstatus &status)
>  {
> @@ -1241,6 +1288,16 @@ prepare_resume_reply (char *buf, ptid_t ptid, const target_waitstatus &status)
>  	    buf += strlen (buf);
>  	    current_process ()->dlls_changed = false;
>  	  }
> +
> +	if (current_thread->tdesc != nullptr
> +	    && current_thread->tdesc != current_process ()->tdesc)
> +	  {
> +	    sprintf (buf, "tdesc:");
> +	    buf += strlen (buf);
> +	    sprintf (buf, "%x", get_tdesc_rsp_id (ptid));
> +	    strcat (buf, ";");
> +	    buf += strlen (buf);
> +	  }
>        }
>        break;
>      case TARGET_WAITKIND_EXITED:
> diff --git a/gdbserver/remote-utils.h b/gdbserver/remote-utils.h
> index cb2d6c346c99..61ef80b4dad7 100644
> --- a/gdbserver/remote-utils.h
> +++ b/gdbserver/remote-utils.h
> @@ -75,4 +75,13 @@ int relocate_instruction (CORE_ADDR *to, CORE_ADDR oldloc);
>  
>  void monitor_output (const char *msg);
>  
> +/* Return the ID as used in the remote protocol for the given target
> +   descriptor.  */
> +
> +unsigned int get_tdesc_rsp_id (const target_desc *tdesc);
> +
> +/* Return the target description corresponding to the remote protocol ID.  */
> +
> +const target_desc *get_tdesc_from_rsp_id (unsigned int id);
> +
>  #endif /* GDBSERVER_REMOTE_UTILS_H */
> diff --git a/gdbserver/server.cc b/gdbserver/server.cc
> index 21fb51a45d16..2d1062f98468 100644
> --- a/gdbserver/server.cc
> +++ b/gdbserver/server.cc
> @@ -976,7 +976,15 @@ handle_general_set (char *own_buf)
>  static const char *
>  get_features_xml (const char *annex)
>  {
> -  const struct target_desc *desc = current_target_desc ();
> +  const struct target_desc *desc;
> +  unsigned int id;
> +
> +  if (strcmp (annex, "target.xml") == 0)
> +    desc = current_target_desc ();
> +  else if (sscanf (annex, "target-id-%u.xml", &id) == 1)
> +    desc = get_tdesc_from_rsp_id (id);
> +  else
> +    desc = nullptr;
>  
>    /* `desc->xmltarget' defines what to return when looking for the
>       "target.xml" file.  Its contents can either be verbatim XML code
> @@ -986,7 +994,7 @@ get_features_xml (const char *annex)
>       This variable is set up from the auto-generated
>       init_registers_... routine for the current target.  */
>  
> -  if (strcmp (annex, "target.xml") == 0)
> +  if (desc != nullptr)
>      {
>        const char *ret = tdesc_get_features_xml (desc);
>  
> @@ -1664,6 +1672,11 @@ handle_qxfer_threads_worker (thread_info *thread, struct buffer *buffer)
>    if (name != NULL)
>      buffer_xml_printf (buffer, " name=\"%s\"", name);
>  
> +  if (thread->tdesc != nullptr
> +      && thread->tdesc != get_thread_process (thread)->tdesc)
> +    buffer_xml_printf (buffer, " tdesc=\"%u\"",
> +		       get_tdesc_rsp_id (thread->tdesc));
> +
>    if (handle_status)
>      {
>        char *handle_s = (char *) alloca (handle_len * 2 + 1);
> diff --git a/gdbserver/server.h b/gdbserver/server.h
> index 7997d1a32e6e..58be5027795b 100644
> --- a/gdbserver/server.h
> +++ b/gdbserver/server.h
> @@ -193,6 +193,10 @@ struct client_state
>    /* If true, memory tagging features are supported.  */
>    bool memory_tagging_feature = false;
>  
> +  /* The target descriptions that have been communicated to the client.  The
> +     index of a target description in this vector is the ID used to reference it
> +     in the remote protocol.  */
> +  std::vector<const target_desc *> tdescs;
>  };
>  
>  client_state &get_client_state ();