I guess you know ;)
e.g. via page table tricks from user space, like the PAE mode on IA32
or via kmap.
> Certainly, when this changes, we can make the interfaces adapt to
I am just curious why you didn't consider that case when designing the
interfaces. Was that a deliberate decision or just an oversight?
[I guess the first, but why?]
> Because of this, for example, the sbus IOMMU stuff on sparc32 still
> uses HIGHMEM exactly because of this pointer limitation. In fact,
> any machine using >4GB of memory currently cannot be supported without
> highmem enabled, which is going to enable bounce buffering in the block
> I/O layer, etc.
That's currently the case, but at least on IA32 the block layer must be
fixed soon because it's a serious performance problem in some cases
(and fixing it is not very hard). Now that will probably first use DAC
and not a IO-MMU, and thus not use the pci mapping API, but I would not be
surprised if people came up with IO-MMU schemes for it too.
[actually most IA32 boxes already have one in form of the AGP GART, it's just
not commonly used for serious things yet]
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