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From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH net-next 3/3] macsec: introduce IEEE 802.1AE driver
Date: Wed, 09 Mar 2016 12:09:37 -0500 (EST)
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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.