Such regions of memory are usually marked as uncachable (UC) or write-through (WT) or sometimes write-combine (WC). These specify what type of caching is allowe

Such regions of memory are usually marked as uncachable (UC) or write-through (WT) or sometimes write-combine (WC). These specify what type of caching is allowed, and if you somehow configure an incorrect caching type you're going to have a bad time.