Debugging a custom kernel

[apelete@seketeli.org (Apelete Seketeli)]

On 29-Jun-11, Christopher Harvey wrote:
> On 06/29/11 16:14, Apelete Seketeli wrote:
> > Hello,
> >
> > I am working on a custom kernel, and I would like to add the necessary
> > support to enable it to boot with qemu.
> When you say "debug inside qemu", do you run
> gdb qemu
> or
> gdb vmlinux

"gdb vmlinux" actually. The focus is on the kernel, to know what going
on during the boot process (since it doesn't boot in qemu).

> > In order to achieve that I am
> > trying to debug it inside qemu by attaching a gdb to it.
> you do this by adding -S -s to the qemu boot parameters.
> the from the gdb shell, target remote :1234.

"qemu -S -kernel bzImage", then, using the monitor inside qemu I start
a gdbserver to which I connect to debug the kernel.

>  > I still can't
> > figure where the boot process is getting stuck with step-by-step
> > execution, but it seems that the last function called is "delay_loop"
> > from arch/x86/lib/delay.c.
> Have you run the backtrace (bt) command from the gdb shell? That should 
> tell you what function is calling the __delay.

I didn't, will try that and see if it helps.

> > That function contains some assembly code, does someone know what it
> > is supposed to do ?
> Probably wait a specific amount of time. Since a compiler optimizes C 
> you can't write an accurate delay in C. The compiler wont optimize the 
> inline assembly.

Okay, so I really need to know which function is calling the delay and
go down from there.

> Have you had any luck with a google search along the lines of "qemu gdb 
> kernel"?

Running gdb with qemu caused me some headache, but I got it, as said
before. Thanks for the tips, I'll try and see if I can get something
useful.

-- 
        Apelete