Re: [PATCH] [Arm] Remove dead FPA code

[John Baldwin <jhb@FreeBSD.org>]

On 10/4/22 10:43 AM, Luis Machado wrote:
> On 10/4/22 18:08, John Baldwin wrote:
>> On 10/4/22 1:43 AM, Luis Machado via Gdb-patches wrote:
>>> On 10/3/22 20:16, Pedro Alves wrote:
>>>> On 2022-09-20 1:30 p.m., Luis Machado via Gdb-patches wrote:
>>>>
>>>>> diff --git a/gdb/arch/arm.h b/gdb/arch/arm.h
>>>>> index 36757493406..74a6ba93bc7 100644
>>>>> --- a/gdb/arch/arm.h
>>>>> +++ b/gdb/arch/arm.h
>>>>> @@ -44,11 +44,6 @@ enum gdb_regnum {
>>>>>       ARM_SP_REGNUM = 13,        /* Contains address of top of stack */
>>>>>       ARM_LR_REGNUM = 14,        /* address to return to from a function call */
>>>>>       ARM_PC_REGNUM = 15,        /* Contains program counter */
>>>>> -  /* F0..F7 are the fp registers for the (obsolete) FPA architecture.  */
>>>>
>>>> Shouldn't we leave behind a comment explaining why there's a hole between 15 and 25?
>>>
>>> I pondered about this a bit more, and I think we should close the gap and bring CPSR down to
>>> 16, its "natural" position. It is what linux uses for user_regs as well, in gdb/arch/arm-linux.h:
>>>
>>> /* The index to access CSPR in user_regs defined in GLIBC.  */
>>> #define ARM_CPSR_GREGNUM 16
>>>
>>>>
>>>> IIRC the numbers can't be changed since we need to handle the case when the target
>>>> doesn't send an xml tdesc, so it'd be good to help future readers understand why
>>>> there's a hole.
>>>
>>> That's correct. Though a 32-bit Arm target that doesn't support XML descriptions these days is not very
>>> common. I haven't seen one in a while.
>>>
>>> I'm willing to declare old 32-bit Arm targets that don't send XML target descriptions back as unsupported.
>>>
>>> To that effect, I suppose we should add a note to make it more explicit.
>>>
>>> More below.
>>
>> FWIW, the GDB stub in FreeBSD's kernel does not use XML target descriptions
>> for any architectures, but it also only tends to do GPRs and not any floating
>> point.  For 32-bit ARM it does not report any register values higher than
>> number 15 (PC), so it would not be affected by changing this.
> 
> Does it care about CPSR and/or XPSR? Could you please give it a try to see if the defaults would suit it just fine?

Hmm, I misread and it does care about CPSR for the current thread.  The
relevant code is here (From https://cgit.freebsd.org/src/tree/sys/arm/arm/gdb_machdep.c):

void *
gdb_cpu_getreg(int regnum, size_t *regsz)
{

	*regsz = gdb_cpu_regsz(regnum);

	if (kdb_thread == curthread) {
		if (regnum < 13)
			return (&kdb_frame->tf_r0 + regnum);
		if (regnum == 13)
			return (&kdb_frame->tf_svc_sp);
		if (regnum == 14)
			return (&kdb_frame->tf_svc_lr);
		if (regnum == 15)
			return (&kdb_frame->tf_pc);
		if (regnum == 25)
			return (&kdb_frame->tf_spsr);
	}

	switch (regnum) {
	case 4:  return (&kdb_thrctx->pcb_regs.sf_r4);
	case 5:  return (&kdb_thrctx->pcb_regs.sf_r5);
	case 6:  return (&kdb_thrctx->pcb_regs.sf_r6);
	case 7:  return (&kdb_thrctx->pcb_regs.sf_r7);
	case 8:  return (&kdb_thrctx->pcb_regs.sf_r8);
	case 9:  return (&kdb_thrctx->pcb_regs.sf_r9);
	case 10:  return (&kdb_thrctx->pcb_regs.sf_r10);
	case 11:  return (&kdb_thrctx->pcb_regs.sf_r11);
	case 12:  return (&kdb_thrctx->pcb_regs.sf_r12);
	case 13:  stacktest = kdb_thrctx->pcb_regs.sf_sp + 5 * 4;
		  return (&stacktest);
	case 15:
		  /*
		   * On context switch, the PC is not put in the PCB, but
		   * we can retrieve it from the stack.
		   */
		  if (kdb_thrctx->pcb_regs.sf_sp > KERNBASE) {
			  kdb_thrctx->pcb_regs.sf_pc = *(register_t *)
			      (kdb_thrctx->pcb_regs.sf_sp + 4 * 4);
			  return (&kdb_thrctx->pcb_regs.sf_pc);
		  }
	}

	return (NULL);
}

The 'kdb_thread == curthread' case is when a thread enters the debugger due
to a crash or breakpoint, etc.  We do return CPSR for that thread, but we do
not return it for other threads.  It looks like we do also know the FPA register
size so that we return enough "xx" bytes in the 'g' reply to mark the FP
registers as unavailable so that we can return the value of CPSR in the 'g'
reply.

 From https://cgit.freebsd.org/src/tree/sys/arm/include/gdb_machdep.h:

#define	GDB_NREGS	26
#define	GDB_REG_SP	13
#define	GDB_REG_LR	14
#define	GDB_REG_PC	15

static __inline size_t
gdb_cpu_regsz(int regnum)
{
	/*
	 * GDB expects the FPA registers f0-f7, each 96 bits wide, to be placed
	 * in between the PC and CSPR in response to a "g" packet.
	 */
	return (regnum >= 16 && regnum <= 23 ? 12 : sizeof(int));
}

NetBSD's kernel seems to have similar knowledge:

http://cvsweb.netbsd.org/bsdweb.cgi/src/sys/arch/arm/include/db_machdep.h?rev=1.28&content-type=text/x-cvsweb-markup&only_with_tag=MAIN

(The kgdb bits near the bottom)

Linux's kernel also seems to maybe hardcode this knowledge as well:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm/kernel/kgdb.c#n21

-- 
John Baldwin