Re: x86 swiotlb questions

["Jan Beulich" <jbeulich@novell.com>]

>>> Keir Fraser <keir@xensource.com> 19.12.06 15:14 >>>
>On 19/12/06 12:48, "Jan Beulich" <jbeulich@novell.com> wrote:
>
>>> Do we merge okay with lib/swiotlb.c then?
>> 
>> Not yet - because of the highmem handling needed for i386. I wonder, however,
>> how native Linux gets away with not handling this through swiotlb, and why
>> nevertheless Xen needs to special case this. Any ideas?
>
>Probably because GFP_KERNEL and GFP_DMA allocations are guaranteed to be
>DMAable by 30-bit-capable devices on native, but not on Xen.

Not sure I understand your thinking here. Nothing prevents user pages (or anything
else that I/O may happen against) to come from highmem, hence the bounce logic
in mm/highmem.c needs to control this anyway (as I understand it). And since all
we're talking about here are physical addresses (and their translations to virtual
ones), I would rather conclude that we'll never see a page in the I/O path that
page_address() would return NULL for, but if that's the case, then there's no need
to kmap such pages or to favor page_to_bus() over virt_to_bus(page_address()).

Jan