I thought you basically create an OperationRegion of SystemMemory type, and off you go. Maybe the OSPM in Linux is clever and protects some memory, I wouldn't know.
I investigated this now, and it looks like acpi is using ioremap_cache(). We can hook into that and force non sharing. It's probably safe to assume that this is not used on real IO devices.
I think there are still some other BIOS mappings that use just plain ioremap() though.
-Andi
