[OpenBIOS] [Qemu-ppc] Running client with MMU off

[BALATON Zoltan]

On Wed, 30 Jul 2014, Alexander Graf wrote:
>> Am 30.07.2014 um 16:42 schrieb BALATON Zoltan <balaton at eik.bme.hu>:
>>>> As a start I've tried this patch:
>>>> --- a/openbios-devel/arch/ppc/qemu/ofmem.c
>>>> +++ b/openbios-devel/arch/ppc/qemu/ofmem.c
>>>> @@ -460,15 +460,26 @@ static void hash_page(unsigned long ea, phys_addr_t phys,
>>>> void
>>>> dsi_exception(void)
>>>> {
>>>> -    unsigned long dar, dsisr;
>>>> +    unsigned long dar, dsisr, srr1;
>>>>     ucell mode;
>>>>     phys_addr_t phys;
>>>>
>>>>     asm volatile("mfdar %0" : "=r" (dar) : );
>>>>     asm volatile("mfdsisr %0" : "=r" (dsisr) : );
>>>> -
>>>> +    asm volatile("mfsrr1 %0" : "=r" (srr1) : );
>>>>     phys = ea_to_phys(dar, &mode);
>>>> -    hash_page(dar, phys, mode);
>>>> +
>>>> +    if (dsisr & BIT(1)) {
>>>> +        /* handle page fault */
>>>> +        hash_page(dar, phys, mode);
>>>> +    }
>>>> +
>>>> +    if (dsisr & BIT(4) && dar == 0) {
>>>> +        /* handle protection violation */
>>>> +        hash_page(dar, phys, mode);
>>>> +        srr1 &= ~(MSR_IR | MSR_DR);
>>>> +        asm volatile("mtsrr1 %0" :: "r" (srr1));
>>>> +    }
>>>> }
>>>>
>>>> void
>>>> @@ -554,9 +565,10 @@ ofmem_init(void)
>>>>     ofmem_claim_virt(PAGE_SIZE, get_ram_bottom() - PAGE_SIZE, 0);
>>>>     ofmem_map(PAGE_SIZE, PAGE_SIZE, get_ram_bottom() - PAGE_SIZE, 0);
>>>> -    /* Mark the first page as non-free */
>>>> +    /* Mark the first page as non-free and write protect it */
>>>>     ofmem_claim_phys(0, PAGE_SIZE, 0);
>>>>     ofmem_claim_virt(0, PAGE_SIZE, 0);
>>>> +    hash_page(0, 0, 3);
>>>>
>>>>     /* Map everything at the top of physical RAM 1:1, minus the OpenBIOS ROM in
>>>>     ofmem_claim_phys(get_ram_top(), get_hash_base() + HASH_SIZE - get_ram_top()
>>>> which does not break Finnix but does not work with MorphOS because it catches the write to 0x80 as it should but instead of ignoring it this protection violation exception is always retrigerring infinitely and it does not go further. What am I missing to ignore protection violations for writes to page 0 without emulating the writes at the moment. (The initial write to 0x80 is setting it to 0 which is the value it already has.) Unfortunatly it is hard to debug because if I call printk from the exception handler it seems to break beyond repair possibly due to side effects.
>>>> I've also looked at the code you've referred to but that uses kvm functions not included in the patch so it may be more complicated than that. If I get it correctly I can get the instruction from the address in srr0, the target memory cell from dar but still need to find out the source which is probably a register that does not contain the value by the time I get it so it may not be trivial to emulate the write.
>>>
>>> They have to be somewhere, because we have to swap them all back in after the interrupt.
>>
>> They are probably on a stack somewhere but the handlers replace the stack if I remember correctly so it's not trivial to access that from C code.
>
> Then pass a pointer to them to the C handler code?

Good idea, but I'd like to make it work first before starting emulating 
instructions.

>> But my question was primarily about why the exception is retriggering? Do I need to do something else to signal to the CPU that the protection violation is handled or how can I ignore an exception so the execution continues?
>
> If you don't fix up the reason why the interrupt occured in the first place it will simply happen again.

I don't get it. The exception happens because of a write to 0x80 which is 
in a protected page. (The value written is the same that is already in 
that memory cell by the way so no action is needed.) What could I do to 
"fix up the reason" so that the exception does not happen again after 
returning from the handler? I thought that simply ignoring the write and 
returning from the handler should make it continue execution but it does 
not seem to work that way. I'm clearly missing something but I don't know 
what. Can you spot a problem in the code above?

Regards,
BALATON Zoltan