[Crash-utility] Re: [PATCH v6 0/5] Improve stack unwind on ppc64

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Hi Kazu,

On Fri, Feb 2, 2024 at 3:04 PM HAGIO KAZUHITO(萩尾 一仁) <k-hagio-ab@xxxxxxx> wrote:
>
> Hi Aditya,
>
> Thank you for the work, it looks nicely done to me, except for the "info
> threads" issue and the style of the gdb patch.
>
> Hi Tao Liu,
>
> I saw that you were interested in support for other architectures.  I'd
> like to test/support this also on x86_64 at the same time as ppc64 if
> possible.  Do you have any trial patch or plan?
>
Yes, I have a trial patch. According to my test, most of the vmcores
can work as expected. But there are some cases which I didn't have a
clue, I can describe these cases in the "Known issue". I planned to
send out the trial patch after the ppc64 patch gets merged, since mine
is dependent on this ppc series.

Please give me a few days before I arrange and post the trial patch.

Thanks,
Tao Liu

> Thanks,
> Kazu
>
> On 2024/01/05 16:30, Aditya Gupta wrote:
> > The Problem:
> > ============
> >
> > Currently crash is unable to show function arguments and local variables, as
> > gdb can do. And functionality for moving between frames ('up'/'down') is not
> > working in crash.
> >
> > Crash has 'gdb passthroughs' for things gdb can do, but the gdb passthroughs
> > 'bt', 'frame', 'info locals', 'up', 'down' are not working either, due to
> > gdb not getting the register values from `crash_target::fetch_registers`,
> > which then uses `machdep->get_cpu_reg`, which is not implemented for PPC64
> >
> > Proposed Solution:
> > ==================
> >
> > Fix the gdb passthroughs by implementing "machdep->get_cpu_reg" for PPC64.
> > This way, "gdb mode in crash" will support this feature for both ELF and
> > kdump-compressed vmcore formats, while "gdb" would only have supported ELF
> > format
> >
> > This way other features of 'gdb', such as seeing
> > backtraces/registers/variables/arguments/local variables, moving up and
> > down stack frames, can be used with any ppc64 vmcore, irrespective of
> > being ELF format or kdump-compressed format.
> >
> > Note: This doesn't support live debugging on ppc64, since registers are not
> > available to be read
> >
> > Implications on Architectures:
> > ====================================
> >
> > No architecture other than PPC64 has been affected, other than in case of
> > 'frame' command
> >
> > As mentioned in patch #2, since frame will not be prohibited, so it will print:
> >
> >       crash> frame
> >       #0  <unavailable> in ?? ()
> >
> > Instead of before prohibited message:
> >
> >       crash> frame
> >       crash: prohibited gdb command: frame
> >
> > Major change will be in 'gdb mode' on PPC64, that it will print the frames, and
> > local variables, instead of failing with errors showing no frame, or showing
> > that couldn't get PC, it will be able to give all this information.
> >
> > Testing:
> > ========
> >
> > Git tree with this patch series applied:
> > https://github.com/adi-g15-ibm/crash/tree/stack-unwind-v6
> >
> > To test various gdb passthroughs:
> >
> >       (crash) set
> >       (crash) set gdb on
> >       gdb> thread
> >       gdb> bt
> >       gdb> info threads
> >       gdb> info threads
> >       gdb> info locals
> >       gdb> info variables irq_rover_lock
> >       gdb> info args
> >       gdb> thread 2
> >       gdb> set gdb off
> >       (crash) set
> >       (crash) set -c 6
> >       (crash) gdb thread
> >       (crash) bt
> >       (crash) gdb bt
> >       (crash) frame
> >       (crash) up
> >       (crash) down
> >       (crash) info locals
> >
> > Known Issues:
> > =============
> >
> > 1. In gdb mode, 'bt' might fail to show backtrace in few vmcores collected
> >     from older kernels. This is a known issue due to register mismatch, and
> >     its fix has been merged upstream:
> >
> >     This can also cause some 'invalid kernel virtual address' errors during gdb
> >     unwinding the stack registers
> >
> > Commit: https://github.com/torvalds/linux/commit/b684c09f09e7a6af3794d4233ef785819e72db79
> >
> > Fixing GDB passthroughs on other architectures
> > ==============================================
> >
> > Much of the work for making gdb passthroughs like 'gdb bt', 'gdb
> > thread', 'gdb info locals' etc. has been done by the patches introducing
> > 'machdep->get_cpu_reg' and this series fixing some issues in that.
> >
> > Other architectures should be able to fix these gdb functionalities by
> > simply implementing 'machdep->get_cpu_reg (cpu, regno, ...)'.
> >
> > The reasoning behind that has been explained with a diagram in commit
> > description of patch #1
> >
> > I will assist with my findings/observations fixing it on ppc64 whenever needed.
> >
> > Changelog:
> > ==========
> >
> > V6:
> > + changes in patch #5: fix bug introduced in v5 that caused initial gdb thread
> >    to be thread 1
> >
> > V5:
> > + changes in patch #1: made ppc64_get_cpu_reg static, and remove unreachable
> >    code
> > + changes in patch #3: fixed typo 'ppc64_renum' instead of 'ppc64_regnum',
> >    remove unneeded if condition
> > + changes in patch #5: implement refresh regcache on per thread, instead of all
> >    threads at once
> >
> > V4:
> > + fix segmentation fault in live debugging (change in patch #1)
> > + mention live debugging not supported in cover letter and patch #1
> > + fixed some checkpatch warnings (change in patch #5)
> >
> > V3:
> > + default gdb thread will be the crashing thread, instead of being
> >    thread '0'
> > + synchronise crash cpu and gdb thread context
> > + fix bug in gdb_interface, that replaced gdb's output stream, losing
> >    output in some cases, such as info threads and extra output in info
> >    variables
> > + fix 'info threads'
> >
> > RFC V2:
> >    - removed patch implementing 'frame', 'up', 'down' in crash
> >    - updated the cover letter by removing the mention of those commands other
> >       than the respective gdb passthrough
> >
> > Aditya Gupta (5):
> >    ppc64: correct gdb passthroughs by implementing machdep->get_cpu_reg
> >    remove 'frame' from prohibited commands list
> >    synchronise cpu context changes between crash/gdb
> >    fix gdb_interface: restore gdb's output streams at end of
> >      gdb_interface
> >    fix 'info threads' command
> >
> >   crash_target.c  |  44 ++++++++++++++++
> >   defs.h          | 130 +++++++++++++++++++++++++++++++++++++++++++++++-
> >   gdb-10.2.patch  | 110 +++++++++++++++++++++++++++++++++++++++-
> >   gdb_interface.c |   2 +-
> >   kernel.c        |  47 +++++++++++++++--
> >   ppc64.c         |  95 +++++++++++++++++++++++++++++++++--
> >   task.c          |  14 ++++++
> >   tools.c         |   2 +-
> >   8 files changed, 434 insertions(+), 10 deletions(-)
> >
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