Re: [Qemu-devel] [RFC] Memory API

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-05-18 17:17, Peter Maydell wrote:
> On 18 May 2011 16:11, Jan Kiszka <jan.kiszka@siemens.com> wrote:
>> On 2011-05-18 16:36, Avi Kivity wrote:
>>> There is nothing we can do with a return code.  You can't fail an mmio
>>> that causes overlapping physical memory map.
>>
>> We must fail such requests to make progress with the API. That may
>> happen either on caller side or in cpu_register_memory_region itself
>> (hwerror). Otherwise the new API will just be a shiny new facade for on
>> old and still fragile building.
> 
> If we don't allow overlapping regions, then how do you implement
> things like "on startup board maps ROM into lower addresses
> over top of devices, but later it is unmapped and you can see
> the underlying devices" ? (You can't currently do this AFAIK,
> and it would be nice if the new API supported it.)

Right, we can't do this properly, and that's why the attempt if the
i440fx chipset model is so horribly broken ATM.

Just allowing overlapping does not solve this problem either. It does
not specify what region is on top and what is below (even worse if
multiple regions overlap at the place).

We need some managing instance here, and that is e.g. the chipset that
provide control over the overlap in reality. It could hook up a
PhysMemClient to receive and redirect updates to subregions, or only
allow to register them in disabled state.

Jan

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