Re: [PATCH v2 3/4] secure_seq: use siphash24 instead of md5_transform

From: "Jason A. Donenfeld" <Jason@zx2c4.com>
Date: Wed, 14 Dec 2016 13:53:10 +0100

In all current uses of __packed in the code, I think the impact is
precisely zero, because all structures have members in descending
order of size, with each member being a perfect multiple of the one
below it. The __packed is therefore just there for safety, in case
somebody comes in and screws everything up by sticking a u8 in
between.

Just marking the structure __packed, whether necessary or not, makes
the compiler assume that the members are not aligned and causes
byte-by-byte accesses to be performed for words.

Never, _ever_, use __packed unless absolutely necessary, it pessimizes
the code on cpus that require proper alignment of types.