Re: [Qemu-devel] [PATCH 0/2] [RFC] 64-bit io paths

[Richard Henderson <rth@twiddle.net>]

On 04/22/2010 12:38 PM, Blue Swirl wrote:
> Subpages are used when there are several devices on the same page.
> It's needed for at least Sparc32.

That's fine.

> Subwidth (with NULL) is used mainly to indicate that the device does
> not accept accesses in some access widths. Sparc32 and Sparc64 need
> this (or some other way to signal bus errors for bad access widths).

This is also fine.  Although by using NULL all you'd get is a qemu
null pointer dereference; I suppose this might get caught and 
translated to an cpu exception, but I think it would be preferable
long-term to be more explicit about this and fill in the entries
with a function that would explicitly raise the exception.

What this *does* confirm for me is that we don't need to support
multiple devices at the same address, differentiated by the size
of the reference.  Which is something that the current subpage
implementation actually supports.

I'll submit a patch to clean this up.

> In fact, many system devices on Sparc64 should only accept 64 bit
> accesses, but currently we can't enforce this.

Cool.  This is something that should be fixed by the end result
of the driver interface re-design that pbrook and I have been
discussing on IRC.


r~