Remote debugging HiFive Unleashed in QEMU - gdb

Question

I'm trying to get remote debugging working in QEMU for the sifive_u machine. All tools are from the Arch Linux repositories:
➜ qemu-system-riscv64 --version
QEMU emulator version 4.2.0
Copyright (c) 2003-2019 Fabrice Bellard and the QEMU Project developers
➜ riscv64-linux-gnu-gdb --version
GNU gdb (GDB) 8.3.1
Copyright (C) 2019 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
I'm starting the machine as follows:
qemu-system-riscv64 -M sifive_u -m 256M -bios default -nographic -S -s
When I connect the debugger, I attempt to continue execution, but nothing happens; if I detach the debugger, the OpenSBI splash prints to the serial console. A typical gdb session looks something like this:
GNU gdb (GDB) 8.3.1
Copyright (C) 2019 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "--host=x86_64-pc-linux-gnu --target=riscv64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word".
(gdb) target remote :1234
Remote debugging using :1234
warning: No executable has been specified and target does not support
determining executable automatically. Try using the "file" command.
0x0000000000001000 in ?? ()
(gdb) info thread
Id Target Id Frame
* 1 Thread 1.1 (sifive-e51-riscv-cpu harts[0] [running]) 0x0000000000001000 in ?? ()
(gdb) c
Continuing.
^C
Program received signal SIGINT, Interrupt.
0x0000000080005a52 in ?? ()
(gdb) info thread
Id Target Id Frame
* 1 Thread 1.1 (sifive-e51-riscv-cpu harts[0] [halted ]) 0x0000000080005a52 in ?? ()
(gdb) detach
Detaching from program: , process 1
Ending remote debugging.
[Inferior 1 (process 1) detached]
It seems odd that I can only see a single thread in info thread; I would expect to see one thread per hart.
My hunch is that I end up attached to a hart which loses the lottery and goes to sleep, and for some none of the other harts are allowed to continue execution. If I use the virt machine, the execution starts as expected when I run continue and I see the OpenSBI splash immediately, so it seems to be linked to the use of the sifive_u in some way.
Does anyone have any idea what I'm doing wrong?

Answer 1

https://www.qemu.org/docs/master/system/gdb.html#Debugging%20multicore%20machines
See "Debugging multicore machines" in this page.
(gdb)target extended-remote :1234
(gdb)add-inferior
(gdb)inferior 2
(gdb)attach 2
(gdb)i threads
See this picture