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From: Thomas Monjalon <thomas.monjalon@6wind.com>
To: Andrey Chilikin <andrey.chilikin@intel.com>
Date: Wed, 03 Jun 2015 10:16:38 +0200
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Cc: dev@dpdk.org
Subject: Re: [dpdk-dev] [PATCH] examples: add ip version check for l3fwd app
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2015-05-15 17:08, Andrey Chilikin:
> Added optional ip version check to l3fwd app to allow to detect
> the ip version if mbuf ol_flags are not set in case of running
> in a VM with emulated network controllers
> 
> Signed-off-by: Andrey Chilikin <andrey.chilikin@intel.com>
[...]
> +#define DO_IP_VERSION_CHECK             0
[...]
> @@ -953,6 +955,15 @@ l3fwd_simple_forward(struct rte_mbuf *m, uint8_t portid, struct lcore_conf *qcon
>  	void *d_addr_bytes;
>  	uint8_t dst_port;
>  
> +#if DO_IP_VERSION_CHECK
> +	if (!(m->ol_flags & (PKT_RX_IPV4_HDR | PKT_RX_IPV6_HDR))) {
> +		uint8_t ip_ver = *(uint8_t *)(rte_pktmbuf_mtod(m, unsigned char *) +
> +			sizeof(struct ether_hdr)) >> 4;
> +		if (ip_ver == 4)
> +			m->ol_flags |= PKT_RX_IPV4_HDR;
> +	}
> +#endif

You are adding dead code. When ol_flags will be updated, it will be forget
until someone enables it.
In general, compile-time configurations are avoided and it's even worst
when this is hidden and not easily testable like here.