Thread awareness is not available on QEMU targets

[bdmendes]

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Describe the bug

gdb is unable to show threads list while debugging an executable on Zephyr QEMU targets. Instead of full thread list only the one running thread (current thread) is shown.

This is only an issue with the various Zephyr QEMU targets; thread awareness works as expected with the hardware targets I have tested, through OpenOCD.

To Reproduce

Steps to reproduce the behavior:

1. west build -b qemu_x86 zephyr/samples/philosophers/
2. ninja debugserver
3. [zephyrsdk path]/x86_64-zephyr-elf/bin/x86_64-zephyr-elf-gdb zephyr/build/zephyr/zephyr.elf
4. Halt execution and issue info threads
5. See only one running thread

Expected behavior

A complete list of all program threads shown, preferably with their name and priority.

Impact

This makes it harder to debug highly concurrent applications for issues such as thread synchronization.

Logs and console output

Expected output (example from board iotdk):

west debug
(gdb) continue
(gdb) ^C
(gdb) info threads
  Id   Target Id                                                                   Frame
* 2    Thread 2147484656 "idle" (Name: idle, prio:40,useropts:1)                   arch_cpu_idle ()
    at /home/mendesb/zephyrproject/zephyr/arch/arc/core/cpu_idle.S:49
  3    Thread 2147484368 "Philosopher 5" (Name: Philosopher 5, prio:-2,useropts:4) 0x00003ce2 in ?? ()
  4    Thread 2147484224 "Philosopher 4" (Name: Philosopher 4, prio:-1,useropts:4) 0x00003ce2 in ?? ()
  5    Thread 2147484080 "Philosopher 3" (Name: Philosopher 3, prio:0,useropts:4)  0x00003ce2 in ?? ()
  6    Thread 2147483936 "Philosopher 2" (Name: Philosopher 2, prio:1,useropts:4)  0x00003ce2 in ?? ()
  7    Thread 2147483792 "Philosopher 1" (Name: Philosopher 1, prio:2,useropts:4)  0x00003ce2 in ?? ()
  8    Thread 2147483648 "Philosopher 0" (Name: Philosopher 0, prio:3,useropts:4)  0x00003ce2 in ?? ()

Actual output from qemu_x86:

(gdb) continue
(gdb) ^C
(gdb) info threads
  Id   Target Id                    Frame
* 1    Thread 1.1 (CPU#0 [running]) 0xe61db94c in ?? ()

Environment

• OS: Ubuntu 22.04
• Toolchain: Zephyr SDK
• Zephyr Commit SHA: fe15f7736c

[bdmendes]

After digging through QEMU's gdbserver (gdbstub.c), it appears to me that the "threads" it exposes are a one-to-one relationship with the emulated cores, and not the target's OS threads. As such, a lot of additional infrastructure needs to be built into QEMU for OS/plataform-specific inspection. https://github.com/zephyrproject-rtos/zephyr/discussions/57816 might be a relevant discussion.

[lmapii]

This is a bit of a bugger right now for debugging integration tests related to threads on anything but Linux since the native_posix target is only supported on Linux. So right now on macos or Windows the only way to get thread awareness when testing is via an actual board (?)