Re: [GXemul-devel] GXemul ARM bkpt patch

[Anders G. <ga...@gm...>]

Tis 2008-06-10 klockan 11:20 +0200 skrev Jiri Svoboda:
> Hello,

Hi Jiri. (I'm sending this reply to gxemul-devel as well, I hope that is
ok with you.)

> I am working on debugging support for the HelenOS operating system as
> a part of my master thesis. While working on this I ran into GXemul
> not supporting the BKPT instruction on ARM (BKPT is defined in ARM
> architecture v5 and above). I made a patch against GXemul 0.4.6.3
> adding support for this instruction. It would be cool if you could
> integrate it (or something equivalent).
...
> /*
>  * bkpt:  Breakpoint instruction.
>  */
> void arm_instr_bkpt(struct cpu *cpu, struct arm_instr_call *ic)
> {
>       /*  Synchronize the program counter first:  */
>       cpu->pc &= 0xfffff000;
>       cpu->pc += ic->arg[0];
>       arm_exception(cpu, ARM_EXCEPTION_DATA_ABT);
> }
... 
>
>               /* "bkpt" */
>               if ((iword & 0xfff000f0) == 0xe1200070) {
>                       ic->f = arm_instr_bkpt;
>                       ic->arg[0] = addr & 0xfff;
>                       break;
>               }

Thanks for your patch. I'm applying it, with some modifications.

The way ARM instructions are implemented in the cpu_arm_instr.c file is
with X() and Y() macros; if you look at e.g. the X(swi) macro call
(which indeed expands to arm_instr_swi(struct cpu *cpu, struct
arm_instr_call *ic)) you will see that it is followed by a Y(swi) as
well.

That is what enables the 4-bit condition code stuff.

Similarly with  ic->f = arm_instr_bkpt;  which does not take the
condition code into account. The cond_instr() macro is used instead.

So the end result is:


/*
 * bkpt:  Breakpoint instruction.
 */
X(bkpt)
{
        /*  Synchronize the program counter first:  */
        cpu->pc &= 0xfffff000;
        cpu->pc += ic->arg[0];
        arm_exception(cpu, ARM_EXCEPTION_PREF_ABT);
}
Y(bkpt)

and

/*  "bkpt", ARMv5 and above  */
if ((iword & 0x0ff000f0) == 0x01200070) {
        ic->arg[0] = addr & 0xfff;
        ic->f = cond_instr(bkpt);
        break;
}


Note: the ARM manual I have here says that the bkpt instruction should
behave as a "Prefetch Abort", not "Data Abort". Are you sure
that ARM_EXCEPTION_DATA_ABT is the right choice? I'm using
ARM_EXCEPTION_PREF_ABT for now (vector 0x000c).

(I also added disassembly support in cpu_arm.c.)

Hopefully I'll have time to make a 0.4.6.4 release soon; I did another
ARM fix a few days ago in the 0.4 branch, which would be good to
release.


Anders