[OpenBIOS] [PATCH] Use exception vectors from ROM instead of copying them to page zero

[Mark Cave-Ayland]

On 26/05/14 13:20, BALATON Zoltan wrote:

> Instead of copying the exception vectors from ROM to page zero use the
> Interrupt Prefix bit of the CPU to use the vectors at the ROM address.
> This simplifies the startup routine a little bit and avoids using
> addresses that can be overwritten by clients. In particular MorphOS
> writes to address 0x80 during boot which corrupted the exception
> return code causing a crash.
>
> Signed-off-by: BALATON Zoltan <balaton at eik.bme.hu>
> ---
>
> Index: openbios-devel/arch/ppc/qemu/start.S
> ===================================================================
> --- openbios-devel/arch/ppc/qemu/start.S	(revision 1304)
> +++ openbios-devel/arch/ppc/qemu/start.S	(working copy)
> @@ -388,7 +388,7 @@ GLOBL(__vectors_end):
>   GLOBL(_entry):
>
>   #ifdef CONFIG_PPC_64BITSUPPORT
> -	li	r0,0
> +	li	r0, MSR_IP
>
>   	lis	r3, 0x8000			/* r1=0x80000000 */
>   	add.	r3,r3,r3			/* r1=r1+r1 (high 32bit !0) */
> @@ -404,32 +404,6 @@ no_64bit:
>
>   real_entry:
>   #endif
> -
> -	/* copy exception vectors */
> -
> -	LOAD_REG_IMMEDIATE(r3, __vectors)
> -	li	r4,0
> -	li	r5,__vectors_end - __vectors + 16
> -	rlwinm	r5,r5,0,0,28
> -1:	lwz	r6,0(r3)
> -	lwz	r7,4(r3)
> -	lwz	r8,8(r3)
> -	lwz	r9,12(r3)
> -	stw	r6,0(r4)
> -	stw	r7,4(r4)
> -	stw	r8,8(r4)
> -	stw	r9,12(r4)
> -	dcbst	0,r4
> -	sync
> -	icbi	0,r4
> -	sync
> -	addi	r5,r5,-16
> -	addi	r3,r3,16
> -	addi	r4,r4,16
> -	cmpwi	r5,0
> -	bgt	1b
> -	isync
> -
>   	bl compute_ramsize
>
>   	/* Memory map:
> @@ -521,13 +495,13 @@ _GLOBAL(call_elf):
>   	LOAD_REG_IMMEDIATE(r5, of_client_callback)	// r5 = callback
>   	li	r6,0			// r6 = address of client program arguments (unused)
>   	li	r7,0			// r7 = length of client program arguments (unused)
> -	li	r0,MSR_FP | MSR_ME | MSR_DR | MSR_IR
> +	li	r0,MSR_FP | MSR_ME | MSR_IP | MSR_DR | MSR_IR
>   	MTMSRD(r0)
>   	blrl
>
>   #ifdef CONFIG_PPC64
>       /* Restore SF bit */
> -    LOAD_REG_IMMEDIATE(r0, MSR_SF | MSR_FP | MSR_ME | MSR_DR | MSR_IR)
> +    LOAD_REG_IMMEDIATE(r0, MSR_SF | MSR_FP | MSR_ME | MSR_IP | MSR_DR | MSR_IR)
>       MTMSRD(r0)
>   #endif
>   	LOAD_REG_IMMEDIATE(r8, saved_stack)		// restore stack pointer

Just as a general comment: I know that some OSs using Sun's PROM expect 
to be able to write to the trap table (which is why it is copied to RAM) 
so you'd need to check this thoroughly. Or, if MSR_IP uses virtual 
addresses you could do some MMU twiddling so that the virtual address 
actually points into a safe area of RAM rather than ROM.

As a starting point, have you tried booting any other OS with this 
change? For my OpenBIOS tests I have a collection of various NetBSD, 
FreeBSD, Linux, Darwin and HelenOS that I use to check for regressions.

This is certainly out of my comfort zone though, so will need a 
review/ack from Alex/Andreas.


ATB,

Mark.