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

[Alexander Graf]

On 27.06.14 23:34, BALATON Zoltan wrote:
> On Thu, 26 Jun 2014, Alexander Graf wrote:
>> On 26.06.14 13:20, BALATON Zoltan wrote:
>>> On Thu, 26 Jun 2014, Alexander Graf wrote:
>>>> You could enable write protection on the first page before you 
>>>> enter the payload. Then you could unprotect it and disable 
>>>> interrupts as soon as someone wrote to it. I guess that'd be a 
>>>> pretty solid hack.
>>>
>>> Good idea, I'll look into this. But 0x00-0xff is reserved for 
>>> operating system use and MorphOS does write to 0x80 before touching 
>>> the vectors (that's why I can't just check from the DSI handler). 
>>> Can I selectively enable writes on a write protected page? (I'll 
>>> need to read about it some more.)
>>
>> Not easily. You could emulate the 0x80 write maybe. Is it too early 
>> if you just disable DR/IR on the 0x80 touch? I don't think any other 
>> OS really accesses these ranges, but only time will tell ;).
>
> 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.


Alex