In general, the processor can issue a Write-Back, WriteEvict,
or an Evict transaction for any cache line that is removed from
the L2 cache. You can use these messages to manage an external snoop
filter. However, the snoop filter logic must not depend on such
a message for every clean line dropped from the processor caches,
because in some circumstances the processor might not signal an
eviction. For example, clean evictions are not guaranteed to occur
in cases involving L1 or L2 tag ECC errors.