Re: [Qemu-devel] [PATCH] 64 bit I/O support v7

[Robert Reif <reif@earthlink.net>]

Paul Brook wrote:
>>> sparc hardware is rather abnormal (for qemu at least) because it cares
>>> what happens when you use the wrong width. Most devices don't care, and
>>> having any NULL functions is liable to introduce significant overhead.
>>>       
>> Ok, so that explains the curious code in m48t59.c:
>> ...
>> So nvram_writeq should be present on non sparc architectures
>> and actually should be doing 8 byte accesses?  How do we handle
>> architecture differences like this?  On sparc, it looks like the
>> sbus controller does this because the actual hardware really
>> only has an 8 bit bus.
>>     
>
> Are there actually any cases where this matters?
>
> My guess is that in pactice we only have certain SPARC devices that need to 
> trap when you do a wrong sized access, and for everything else you're told 
> not to do that, and qemu can happily return garbage.
>
> If this is the case then the IO_MEM_SUBWIDTH code seems like complete 
> overkill. I reccommend ripping it out, and maybe having the registration 
> function replace NULL with the unassigned hander.
>
> Paul
>
>
>
>   
Wouldn't making a version of the subpage structure the default
and getting rid of the multiple parallel arrays make more sense.
Then there would only be one type of I/O memory and no special
cases.