Re: Running programs on aarch64 simulator

[Simon Marchi <simark@simark.ca>]

On 2020-05-08 9:44 a.m., Joel Sherrill wrote:
> Hi
> 
> Can someone please provide an example of how to compile a program to run on
> the aarch64 simulator in gdb? The simple "aarch64-elf main.c -o main" does
> not link and appears to be missing libgloss type symbols.
> 
> I asked on newlib but got no response. I'm hoping someone here knows.
> 
> Thanks.
> 
> --joel
> RTEMS
> 

Hi Joel,

If I just compile a "main" function with the bare-metal aarch64 or arm compiler, it does indeed
complain about a missing _exit function:

<snip>/gcc/aarch64-none-elf/9.2.1/../../../../aarch64-none-elf/lib/libg.a(lib_a-exit.o): in function `exit':
<snip>/build/src/newlib-cygwin/newlib/libc/stdlib/exit.c:64: undefined reference to `_exit'

I think this is expected, as the runtime requires you to provide some implementation for _exit,
in other words what you want to do when the program has finished.  Maybe shut down the board,
reset it, shut down the simulator, that's use case-specific.  libgloss indeed provides an
implementation, so I guess you could include that in your program:

  https://sourceware.org/git/?p=newlib-cygwin.git;a=blob;f=libgloss/aarch64/_exit.c;h=16564bbdaaaa3fa20c68198903432dfbf7f768a7;hb=HEAD

But for the GDB simulator, you can just provide a simple implementation of _exit.  In my case,
I just defined it as an empty loop, but maybe there's a way to signal the simulator to shut
down, I don't know.

---
void _exit(int status)
{
  for (;;);
}

int x = 0;

int main(void)
{
  for (int i = 0; i < 1000; i++) {
      x++;
  }
  return 0;
}
---

Compiled with:

$ aarch64-none-elf-gcc test.c -g3 -O0 -o test

or

$ arm-none-eabi-gcc test.c -g3 -O0 -o test

With my arm binary, here's how I can run it with the simulator:

$ ./gdb -nx -q --data-directory=data-directory ./test -ex "target sim" -ex "load" -ex "b main" -ex "run"
Reading symbols from ./test...
Connected to the simulator.
Loading section .init, size 0x18 lma 0x8000
Loading section .text, size 0x630 lma 0x8018
Loading section .fini, size 0x18 lma 0x8648
Loading section .rodata, size 0x4 lma 0x8660
Loading section .ARM.exidx, size 0x8 lma 0x8664
Loading section .eh_frame, size 0x4 lma 0x866c
Loading section .init_array, size 0x4 lma 0x18670
Loading section .fini_array, size 0x4 lma 0x18674
Loading section .data, size 0x430 lma 0x18678
Start address 0x8128
Transfer rate: 21824 bits in <1 sec.
Breakpoint 1 at 0x824c: file test.c, line 10.
Starting program: /home/smarchi/build/binutils-gdb-target-arm/gdb/test

Breakpoint 1, main () at test.c:10
10        for (int i = 0; i < 1000; i++) {
(gdb) n
11            x++;
(gdb)
10        for (int i = 0; i < 1000; i++) {

I tried to build a GDB for the AArch64 target, but it did not include the simulator.  I then
did the following change to enable it:

diff --git a/gdb/configure.tgt b/gdb/configure.tgt
index b3f31af763c..1dcd153f52f 100644
--- a/gdb/configure.tgt
+++ b/gdb/configure.tgt
@@ -114,6 +114,7 @@ case "${targ}" in
 aarch64*-*-elf | aarch64*-*-rtems*)
        # Target: AArch64 embedded system
        gdb_target_obs="aarch64-newlib-tdep.o"
+       gdb_sim=../sim/aarch64/libsim.a
        ;;

This got me a GDB for the AArch64 target with simulator support.  However, it does not
run correctly:

$ ./gdb -nx -q --data-directory=data-directory ./test -ex "target sim" -ex "load" -ex "b main" -ex "run"
Reading symbols from ./test...
Connected to the simulator.
Loading section .init, size 0x34 lma 0x400000
Loading section .text, size 0x67c lma 0x400040
Loading section .fini, size 0x34 lma 0x4006bc
Loading section .rodata, size 0x58 lma 0x4006f0
Loading section .eh_frame, size 0x4 lma 0x400748
Loading section .init_array, size 0x8 lma 0x410750
Loading section .fini_array, size 0x8 lma 0x410758
Loading section .data, size 0x758 lma 0x410760
Start address 0x400168
Transfer rate: 30016 bits in <1 sec.
Breakpoint 1 at 0x4001e8: file test.c, line 10.
Starting program: /home/smarchi/build/binutils-gdb-target-aarch64/gdb/test
core: 8 byte write to unmapped address 0xfffffff0 at 0x0

Program received signal SIGSEGV, Segmentation fault.
0x0000000000000000 in ?? ()


I did not push the investigation further.

Simon