RE: [PATCH v6 net-next 1/2] net: add skb_mstamp infrastructure

From: David Laight <David.Laight@ACULAB.COM>
To: 'Eric Dumazet' <eric.dumazet@gmail.com>,
	Stephen Hemminger <stephen@networkplumber.org>
Cc: Julian Anastasov <ja@ssi.bg>, David Miller <davem@davemloft.net>,
	"Yuchung Cheng" <ycheng@google.com>,
	netdev <netdev@vger.kernel.org>,
	Neal Cardwell <ncardwell@google.com>,
	Larry Brakmo <brakmo@google.com>
Subject: RE: [PATCH v6 net-next 1/2] net: add skb_mstamp infrastructure
Date: Tue, 25 Feb 2014 09:51:45 +0000	[thread overview]
Message-ID: <063D6719AE5E284EB5DD2968C1650D6D0F6CA6D1@AcuExch.aculab.com> (raw)
In-Reply-To: <1393309324.2316.119.camel@edumazet-glaptop2.roam.corp.google.com>

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