cannot set breakpoint at _el2_entry when using gdb debug of bao

[xioliu]

Hello,

I am running the baremetal demo of bao, when I tried to set the breakpoint to _el2_entry or 0xfd8000000000, gdb cannot stop at the place. I found the earliest breakpoint I can set successfully is init in init.c.
Could you please give me any clue that I can debug the boot code from _el2_entry?
Thank you.

[DavidMCerdeira]

Hi,

You are not able to set the breakpoint at that point because those virtual memory memory addresses are only used starting from the init in init.c

The way to debug that code is to put a breakpoint on the physical address bao is running on.
If you are following these steps you should put a breakpoint at address 0x50000000

Let me know if this helps.

[xioliu]

Hello David,

Thank you.
I believe 0x50000000 is the address of _start in bare metal image, and yes, I can set breakpoint at 0x50000000 if I use "aarch64-none-elf-gdb baremetal.elf --tui" to debug. However, I want to debug the bao.elf and start from _el2_entry.

[DavidMCerdeira]

That should not be an issue for your purposes. Setting the breakpoint at 0x50000000 may cause the execution to break in the baremetal app but it will also break the execution at _el2_entry.

I'm assuming you are trying to use the _el2_entry symbol to create the breakpoint. This will not work, as the generated elf will set this symbol address as part of the virtual address space, which is only set just before the init.
You should set the breakpoint in gdb with break *0x50000000 or break *(unsigned long*)0x50000000 depending on gdb version.
This will break execution and let you see the disassembly.

If you want to sync with assembly source code you will have to tell gdb the address where you want to load the symbols to.
Let me know if you need help doing this.

[xioliu]

I tried below commands:
export PATH=$PATH:/home/xioliu/arm-gnu-toolchain-13.2.Rel1-x86_64-aarch64-none-elf/bin
aarch64-none-elf-gdb bao.elf --tui

In gdb:
b *0x50000000
add-symbol-file bao.elf 0x50000000
add-symbol-file baremetal.elf 0x50000000
info b # showed the breakpoint in exceptions.S:71, which means vcpu_arch_entry.

I don't understand why the break point is set here. Though after running, it did stop here.
Another thing is, the address of registers are not as defined in bao.elf.txt.

Could you explain more? Thank you.

[DavidMCerdeira]

Greetings,

Sure.
I'll try to reproduce your case by the end of the week to get you accurate feedback.