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

[Paul Brook]

> > 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