Re: [PATCH 4/5] mbuf: add userdata pointer field

[Neil Horman <nhorman-2XuSBdqkA4R54TAoqtyWWQ@public.gmane.org>]

On Wed, Sep 17, 2014 at 11:01:41AM +0100, Bruce Richardson wrote:
> While some applications may store metadata about packets in the packet
> mbuf headroom, this is not a workable solution for packet metadata which
> is either:
> * larger than the headroom (or headroom is needed for adding pkt headers)
> * needs to be shared or copied among packets
> 
> To support these use cases in applications, we reserve a general
> "userdata" pointer field inside the second cache-line of the mbuf. This
> is better than having the application store the pointer to the external
> metadata in the packet headroom, as it saves an additional cache-line
> from being used.
> 
> Apart from storing metadata, this field also provides a general 8-byte
> scratch space inside the mbuf for any other application uses that are
> applicable.
> 
> Signed-off-by: Bruce Richardson <bruce.richardson-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
> ---
>  lib/librte_eal/linuxapp/eal/include/exec-env/rte_kni_common.h | 3 ++-
>  lib/librte_mbuf/rte_mbuf.h                                    | 3 +++
>  2 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/librte_eal/linuxapp/eal/include/exec-env/rte_kni_common.h b/lib/librte_eal/linuxapp/eal/include/exec-env/rte_kni_common.h
> index 25ed672..d27e891 100644
> --- a/lib/librte_eal/linuxapp/eal/include/exec-env/rte_kni_common.h
> +++ b/lib/librte_eal/linuxapp/eal/include/exec-env/rte_kni_common.h
> @@ -117,7 +117,8 @@ struct rte_kni_mbuf {
>  	uint16_t data_len;      /**< Amount of data in segment buffer. */
>  	uint32_t pkt_len;       /**< Total pkt len: sum of all segment data_len. */
>  	char pad3[8];
> -	void *pool __attribute__((__aligned__(64)));
> +	void *pad4 __attribute__((__aligned__(64)));
> +	void *pool;
I don't see a comment about this in the changelog, only about the userdata
pointer being added below.


>  	void *next;
>  };
>  
> diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> index 8e27d2e..b1acfc3 100644
> --- a/lib/librte_mbuf/rte_mbuf.h
> +++ b/lib/librte_mbuf/rte_mbuf.h
> @@ -172,6 +172,9 @@ struct rte_mbuf {
>  
>  	/* second cache line - fields only used in slow path or on TX */
>  	MARKER cacheline1 __rte_cache_aligned;
> +
> +	void *userdata;           /**< Can be used for external metadata */
> +
Do you want to make this a void* or a char[8]?  I ask because if people are
going to use is as a scratch space (rather than a pointer), they get a suprise
when they build this on 32 bit systems, and their 8 byte scratch space is
reduced to 4 bytes.

Neil

>  	struct rte_mempool *pool; /**< Pool from which mbuf was allocated. */
>  	struct rte_mbuf *next;    /**< Next segment of scattered packet. */
>  
> -- 
> 1.9.3
> 
>