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

Andrew Burgess via Gdb-patches
2023-02-01T14:52:22Z

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 ();