From: Johannes Berg <johannes@sipsolutions.net>
Date: Wed, 09 Mar 2016 12:24:20 +0100

Yeah, there's probably no way a compiler could ever do something with
it that's not the same as packed, but it seems to me that just out of
convention structs that have some wire-format meaning should usually be
__packed.

Not unless absolutely necessary.

There is a serious performance penalty for using __packed on some
architectures.  __packed has several undesirable side-effects, one
of which is that it causes the compiler to be unable to assume the
alignment of anything.