On 2013-08-05 13:35, Andreas Färber wrote:
> Am 05.08.2013 13:03, schrieb Jan Kiszka:
>> On 2013-08-05 12:51, Peter Maydell wrote:
>>> On 5 August 2013 11:44, Jan Kiszka <jan.kiszka@web.de> wrote:
>>>> On 2013-08-05 12:36, Peter Maydell wrote:
>>>>> On 5 August 2013 11:30, Jan Kiszka <jan.kiszka@web.de>
>>>>> wrote:
>>>>>> On 2013-08-05 11:59, Peter Maydell wrote:
>>>>>>> Or do you mean that if we had:
>>>>>>>
>>>>>>> [ system memory region, with its own default read/write
>>>>>>> ops ]
>>>>>>
>>>>>> I cannot imagine how this could work. The system memory
>>>>>> region has no clue about what the regions below it can
>>>>>> handle and what not. So it has to pass through the io
>>>>>> window.
>>>>>
>>>>> The system memory region's just a container, you can add a 
>>>>> background region to it at lowest-possible-priority, which 
>>>>> then takes accesses which are either (a) not in any
>>>>> subregion or (b) in a subregion but that container doesn't
>>>>> specify its own io ops and nothing in that container handles
>>>>> the access. (Or you could create the system memory region
>>>>> with its own IO ops, which would have the same effect.)
>>>>
>>>> First, we do not render MMIO and IO within the same address
>>>> space so far.
>>>
>>> Is this a statement made because you've checked all the boards 
>>> and know that nobody's mapping the system-io memory region into 
>>> the system-memory region? (It is technically trivial, you just
>>> need to call memory_region_add_subregion() directly or
>>> indirectly...)
> 
>> I know this because I just recently wrote the patch that enables
>> this trivial step, i.e. converted PIO dispatching to the memory
>> subsystem.
> 
> Several patches have been applied since, e.g.
> 
> sPAPR PHB:
> http://git.qemu.org/?p=qemu.git;a=commit;h=66aab867cedd2a2d81b4d64eff7c3e0f6f272bbf
> -> aliases system_io()
> 
> PReP i82378 PCI-ISA bridge:
> http://git.qemu.org/?p=qemu.git;a=commit;h=5c9736789b79ea49cd236ac326f0a414f63b1015
> -> uses pci_address_space_io()
> 
> Alpha Typhoon:
> http://git.qemu.org/?p=qemu.git;a=commit;h=056e6bae1c91f47165d962564f82f5176bae47f0
> http://git.qemu.org/?p=qemu.git;a=commit;h=3661049fec64ffd7ab008e57e396881c6a4b53a4
> 
> [For those joining late, this discussion is about whether making PIO
> MemoryRegion ..._io rather than just container might hurt some use
> case. If you have a concrete test case that would be appreciated; a
> we-don't-care-about-such-a-fringe-case would help as well.]

OK, one assumption became outdated, but the other will remain true once
the patch is applied. So let's close this discussion.

Jan