Re: [Qemu-devel] [PATCH] mips/malta: prevent writes to reset flash mapping faulting

[James Hogan <james.hogan@imgtec.com>]

Hi Andreas,

On 23/08/13 12:08, Andreas Färber wrote:
> Am 23.08.2013 09:59, schrieb Leon Alrae:
>> From: James Hogan <james.hogan@imgtec.com>
>>
>> Commit a427338 (mips_malta: correct reading MIPS revision at 0x1fc00010)
>> altered the behaviour of the monitor flash mapping at the reset address
>> by making it read only. However this causes data bus error exceptions
>> when it is written to since it is effectively unassigned memory for
>> writes. This isn't how the real hardware behaves. That memory can be
>> written to (even with the MFWR jumper not fitted) and the new value read
>> back from, but it doesn't get written back to the monitor flash so is
>> volatile.
>>
>> This is fixed by converting the bios copy from read only ram to a bios
>> device with a nop write callback.
> 
> That sounds like a contradiction: The nop write will not have reads
> return the new value, will it?

correct.

> Why not just remove the _set_readonly and have it reloaded on reset for
> volatility?

That's what I tried first, but the bios copy is normal ram so it doesn't
get reloaded on reset. I'll have a play to see if I can use rom_add_blob
(although I seem to remember already trying that...).

> Anyway, having a MemoryRegionOps with just a .write looks dangerous, but
> I guess you've tested read to work. We had been seeing assertions
> elsewhere when either was missing.

Yeh reads seem to work fine (it also executes from it fine).

Thanks for taking a look

James