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

[Luca]

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.
>
> Sure:
>
> (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
> ../i386/i386at/model_dep.c:227
> 227  asm volatile ("hlt" : : : "memory");
> (gdb) bt
> #0  0xffffffff8100c065 in machine_idle (cpu=cpu@entry=0) at
> ../i386/i386at/model_dep.c:227
> #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
> (gdb)

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.


Luca