From mboxrd@z Thu Jan  1 00:00:00 1970
From: Thomas Monjalon <thomas.monjalon@6wind.com>
Subject: Re: [PATCH v7 2/2] net/ixgbe: add callback to user app
	on VF to PF mbox msg
Date: Sat, 08 Oct 2016 00:44:19 +0200
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References: <1475772490-10491-1-git-send-email-bernard.iremonger@intel.com>
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Cc: dev@dpdk.org, rahul.r.shah@intel.com, wenzhuo.lu@intel.com, az5157@att.com
To: Bernard Iremonger <bernard.iremonger@intel.com>
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2016-10-07 17:46, Bernard Iremonger:
> +struct rte_pmd_ixgbe_mb_event_param {
> +	uint16_t vfid;     /**< Virtual Function number */
> +	uint16_t msg_type; /**< message type */
> +	uint16_t retval;   /**< return value */
> +	void *userdata;    /**< pointer to user data */

Generally speaking, the user data is a pointer passed by the application
when registering the callback and must be untouched.
It should be the name of the parameter that you are overriding with
this structure.
So this "userdata" pointer could probably be better defined.

By the way, it is far from trivial to understand how to write the callback.
I think it deserves more explanations.