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

Pedro Alves
2023-02-06T19:11:51Z

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
>