Cache

Write-through

Write-back

The value kept in the backing store should be up-to-
date with any changes made to the version stored
in the cache, but only to the point that it is possible
to ensure that any read requests to the version in the
backing store (e.g., from another processor) return
the most recently written value stored in the cache.

Two typical mechanisms used in general-purpose
caches to effect this responsibility are write-back and
write-through policies. When writing to a cache, the
backing store is implicitly updated, but when? Two
obvious choices are immediately and later; the first
is write-through, and the second is write back.

outer_inv - /* invalidate only */
outer_clean - /* writeback only */
outer_flush - /* writeback and invalidate */