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Re: [RFC PATCH 00/34] The rest of the x86_64-gnu port

From: Luca
Subject: Re: [RFC PATCH 00/34] The rest of the x86_64-gnu port
Date: Tue, 21 Mar 2023 10:48:21 +0100

Il 21/03/23 10:11, Sergey Bugaev ha scritto:
Can you attach with gdb at this point? IIRC you can also dynamically
enable/disable the gdb server through the qemu monitor, or stop and see
the registers from there.


(gdb) target rem :1234
Remote debugging using :1234
warning: Remote gdbserver does not support determining executable automatically.
RHEL <=6.8 and <=7.2 versions of gdbserver do not support such
automatic executable detection.
The following versions of gdbserver support it:
- Upstream version of gdbserver (unsupported) 7.10 or later
- Red Hat Developer Toolset (DTS) version of gdbserver from DTS 4.0 or
later (only on x86_64)
- RHEL-7.3 versions of gdbserver (on any architecture)
0xffffffff8100c065 in machine_idle (cpu=cpu@entry=0) at
227  asm volatile ("hlt" : : : "memory");
(gdb) bt
#0  0xffffffff8100c065 in machine_idle (cpu=cpu@entry=0) at
#1  0xffffffff81019737 in idle_thread_continue () at ../kern/sched_prim.c:1658
#2  0xffffffff81038b8c in Thread_continue () at ../x86_64/cswitch.S:104
#3  0xffffffffbf11ca88 in ?? ()
#4  0xffffffff81018db0 in ?? () at ../kern/sched_prim.c:924
#5  0xffffffffbf117fb8 in ?? ()
#6  0x0000000000000000 in ?? ()
(gdb) info thread
   Id   Target Id                    Frame
* 1    Thread 1.1 (CPU#0 [halted ]) 0xffffffff8100c065 in machine_idle
(cpu=cpu@entry=0) at ../i386/i386at/model_dep.c:227

Great! if you still have issues with the long mode, you could try to hack gnumach to wait at some point (e.g. user_bootstrap() or setup_main()), attach with gdb and then continue. Either with some delay or polling a flag that you flip from gdb once attached. In this way you should be able to set a breakpoint in userspace.

I've no idea how to use kdb anyway, and why it is even needed when GDB
remote exists. (Maybe it's helpful for running on physical hardware?)

It's definitely useful on real hw, and it also allows you to list threads, tasks and many other gnumach-specific stuff. When using gdb you could add some commands using the gdb-python interface to do similar stuff, but it's probably not very useful in this case.

By the way, It seems kdb still needs some work for x86_64, I tried using it to stop execution and resume (as in prompt-task-resume with kdb enabled) and resuming seems to fail.


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