netdev - RE: [PATCH v6 net-next 1/2] net: add skb

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Date:	Tue, 25 Feb 2014 09:51:45 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Eric Dumazet' <eric.dumazet@...il.com>,
	Stephen Hemminger <stephen@...workplumber.org>
CC:	Julian Anastasov <ja@....bg>, David Miller <davem@...emloft.net>,
	"Yuchung Cheng" <ycheng@...gle.com>,
	netdev <netdev@...r.kernel.org>,
	Neal Cardwell <ncardwell@...gle.com>,
	Larry Brakmo <brakmo@...gle.com>
Subject: RE: [PATCH v6 net-next 1/2] net: add skb_mstamp infrastructure

From: Eric Dumazet
> ktime_get() is too expensive on some cases, and we'd like to get
> usec resolution timestamps in TCP stack.
> 
> This patch adds a light weight facility using a combination of
> local_clock() and jiffies samples.
...
> +struct skb_mstamp {
> +	union {
> +		u64		v64;
> +		struct {
> +			u32	stamp_us;
> +			u32	stamp_jiffies;
> +		};
> +	};
> +};

Do you need the union here?
If you don't attempt to convert the value to a u64 then code
is less likely to process it incorrectly.

Most modern ABI pass/return short structures in registers,
so the actual code should be similar if you return the struct
by value.

	David