Re: [PATCH v3 0/8] gdbserver improvements for AArch64 SVE support

[Pedro Alves <pedro@palves.net>]

On 2023-01-30 4:45 a.m., Thiago Jung Bauermann via Gdb-patches wrote:
> Hello,
> 
> This is version 3 of the patch series adding support to gdbserver for
> inferiors that change the SVE vector length at runtime. This is already
> supported by GDB, but not by gdbserver. Version 2 was posted here:
> 
> https://inbox.sourceware.org/gdb-patches/20221126020452.1686509-1-thiago.bauermann@linaro.org/
> 
> This version incorporates the review comments from v2 (thanks!). The
> biggest change is that it implements Simon's idea¹:
> 
>> If it's the case, that we need to extend the RSP to allow fetching
>> per-thread tdesc, here's the idea I had in mind for a while, to avoid
>> adding too much overhead.  Stop replies and the qXfer:threads:read reply
>> could include a per-thread tdesc identifier.  That tdesc identifier
>> would be something short, either an incrementing number, or some kind of
>> hash of the tdesc.  It would be something opaque chosen by the stub.  A
>> new packet would be introduced to allow GDB to request the XML given
>> that ID.  On the GDB side, GDB would request the XML when encountering a
>> tdesc ID it doesn't know about, and then cache the result.
> 
> The only difference from the above in this series is that instead of
> creating a new packet to allow GDB to request the XML given the ID, I'm
> extending the qXfer:features:read request: GDB can send "target-id-%u.xml"
> as the annex, where %u is the target description ID. Please let me know if
> you think a separate packet would be better. The remote protocol changes
> are documented in patch 5.
> 
> I had to drop the Reviewed-by tags from some patches because they have some
> significant changes from the version that was reviewed.
> 
> There's also a new testcase exercising the case of an inferior's thread
> changing its SVE vector length. The previous version of this series failed
> the testcase, but this one passes it.
> 
> With this series applied, gdb.arch/aarch64-sve.exp passes all tests on
> extended-remote aarch64-linux. Without them, it fails tests after the
> testcase changes the vector length.
> 
> Tested on native and extended-remote aarch64-linux, x86_64-linux and
> armv7l-linux-gnueabihf (the last one on QEMU TCG).
> 
> ¹ https://inbox.sourceware.org/gdb-patches/559069a3-f3ce-2059-bf4a-44add43979f7@simark.ca/
> 

Ah!  I had also suggested to you something like that at the Cauldron (when we were in line
for lunch.  :-D  However, IIRC, I had suggested that we should be able to cache the tdesc by filename.

Let me explain -- when we fetch a target description, we actually tell the server
to retrieve a tdesc _by a given filename_.  By default, we ask for target.xml, like this:

 qXfer:features:read:target.xml

but then, the retrieved target.xml file may "xi:include" some other file, like for example these do:

 gdb/features/s390-linux64.xml:14:  <xi:include href="s390-core64.xml"/>
 gdb/features/s390-linux64.xml:15:  <xi:include href="s390-acr.xml"/>
 gdb/features/s390-linux64.xml:16:  <xi:include href="s390-fpr.xml"/>

(try grepping for xi:include in the gdb/features/ dir for a lot more hits.)

and so when processing each of those xi:include's, gdb sends another qXfer:features:read packet,
with the corresponding included filename, like e.g., 

  qXfer:features:read:s390-core64.xml

Here's what the manual says:

 @item qXfer:features:read:@var{annex}:@var{offset},@var{length}
 @anchor{qXfer target description read}
 Access the @dfn{target description}.  @xref{Target Descriptions}.  The
 annex specifies which XML document to access.  The main description is
 always loaded from the @samp{target.xml} annex.

So basically I am suggesting that instead a new ID mechanism, we should be able to
use the preexisting annex/filename concept as key.  That means that the stop reply and
the thread listing would include a new "tdesc=foo.xml" attribute, instead of this
ID that then is defined to map to "target-id-%u.xml", which is basically admitting
that tdesc filenames exist anyhow.

I admit I haven't yet read the patches at all yet (of any series version), but
I'd like bring this up already, in case it helps with saving wasted effort.

Pedro Alves

> Changes since v2:
> 
> - Patch “gdbserver: Add assert in find_register_by_number”
>   - Rewritten to follow Simon's suggestion of putting the assert in another
>     function.
> - Patch “gdbserver: Add PID parameter to linux_get_auxv and linux_get_hwcap”
>   - As suggested by Simon, directly access thread_info::id rather than use
>     pid_of.
>   - Also as suggested by Simon, updated doc comment of
>     process_stratum_target::read_auxv.
> - Patch “gdbserver/linux-aarch64: Factor out function to get aarch64_features”
>   - As suggested by Simon, directly access thread_info::id rather than use
>     lwpid_of.
>   - Also as suggested by Simon, added doc comment for
>     aarch64_get_arch_features.
> - Patch “gdbserver/linux-aarch64: When thread stops, update its target description”
>   - As suggested by Luis, added doc comments to thread_info::tdesc and
>     arch_process_info::has_sve.
>   - As suggested by Simon, renamed arch_update_tdesc to get_thread_tdesc,
>     and return a target_desc pointer (or nullptr) instead of one wrapped in
>     gdb::optional.  The code was changed a bit as well.
>   - As suggested by Luis, expanded doc comment of get_thread_tdesc.
>   - As suggested by Luis, abstracted away trying to fetch a tdesc from a
>     thread and then from a process to a new function
>     "get_thread_target_desc".
>   - Export free_register_cache_thread in gdbserver/regcache.h.
>   - Use free_register_cache_thread in linux_process_target::filter_event to
>     implement Simon's suggestion of only freeing the regcache for the
>     thread with a different tdesc.
> - “gdb/aarch64: Factor out most of the thread_architecture method”
>   - Dropped this patch. It's not needed anymore.
> - “gdbserver: Transmit target description ID in thread list and stop reply”
>   - New patch.
> - “gdb/remote: Parse and use tdesc field in stop reply and threads list XML”
>   - New patch.
> - “gdb/aarch64: Detect vector length changes when debugging remotely”
>   - As suggested by Luis, clarified in the patch description that this patch improves
>     debugging programs remotely. Also, reworded description somewhat.
>   - As suggested by Luis, changed VL references in comments to VG. Also reworded a bit the
>     description of the update_architecture gdbarch method.
>   - Changed the update_architecture gdbarch method (and consequently also
>     aarch64_update_architecture) to take a target description instead of a vector of
>     cached_reg_t.
>   - Changed remote_target::update_thread_list to get the thread target description from
>     the remote target.
>   - Changed remote_target::process_stop_reply to get the thread target description from
>     the remote target.
>   - Renamed remote_thread_info::expedited_arch to arch.
>   - Changed code of remote_target::thread_architecture to be a little bit clearer, and
>     added doc comment.
> - “gdb/testsuite: Add test to exercise multi-threaded AArch64 SVE inferiors”
>   - New patch.
> 
> Thiago Jung Bauermann (8):
>   gdbserver: Add assert in find_register_by_number
>   gdbserver: Add PID parameter to linux_get_auxv and linux_get_hwcap
>   gdbserver/linux-aarch64: Factor out function to get aarch64_features
>   gdbserver/linux-aarch64: When thread stops, update its target
>     description
>   gdbserver: Transmit target description ID in thread list and stop
>     reply
>   gdb/remote: Parse tdesc field in stop reply and threads list XML
>   gdb/aarch64: Detect vector length changes when debugging remotely
>   gdb/testsuite: Add test to exercise multi-threaded AArch64 SVE
>     inferiors
> 
>  gdb/aarch64-tdep.c                            |  20 +++
>  gdb/arch-utils.c                              |   8 +
>  gdb/arch-utils.h                              |   4 +
>  gdb/doc/gdb.texinfo                           |  27 ++-
>  gdb/features/threads.dtd                      |   1 +
>  gdb/gdbarch-components.py                     |  15 ++
>  gdb/gdbarch-gen.h                             |  10 ++
>  gdb/gdbarch.c                                 |  22 +++
>  gdb/remote.c                                  | 169 +++++++++++++++++-
>  gdb/testsuite/gdb.arch/aarch64-sve-threads.c  | 125 +++++++++++++
>  .../gdb.arch/aarch64-sve-threads.exp          |  70 ++++++++
>  gdb/xml-tdesc.c                               |  27 ++-
>  gdb/xml-tdesc.h                               |   6 +
>  gdbserver/gdbthread.h                         |   4 +
>  gdbserver/linux-aarch64-low.cc                |  68 +++++--
>  gdbserver/linux-arm-low.cc                    |   2 +-
>  gdbserver/linux-low.cc                        |  35 +++-
>  gdbserver/linux-low.h                         |  15 +-
>  gdbserver/linux-ppc-low.cc                    |   6 +-
>  gdbserver/linux-s390-low.cc                   |   2 +-
>  gdbserver/netbsd-low.cc                       |   4 +-
>  gdbserver/netbsd-low.h                        |   2 +-
>  gdbserver/regcache.cc                         |  14 +-
>  gdbserver/regcache.h                          |   4 +
>  gdbserver/remote-utils.cc                     |  57 ++++++
>  gdbserver/remote-utils.h                      |   9 +
>  gdbserver/server.cc                           |  20 ++-
>  gdbserver/server.h                            |   4 +
>  gdbserver/target.cc                           |   4 +-
>  gdbserver/target.h                            |   4 +-
>  gdbserver/tdesc.cc                            |  13 +-
>  gdbserver/tdesc.h                             |   5 +
>  32 files changed, 717 insertions(+), 59 deletions(-)
>  create mode 100644 gdb/testsuite/gdb.arch/aarch64-sve-threads.c
>  create mode 100644 gdb/testsuite/gdb.arch/aarch64-sve-threads.exp
> 
> 
> base-commit: 6c76a6beade05f8b2ca93e939cf73da2917d379b
>