Re: [dpdk-dev] [PATCH v8 01/21] mem: add length to memseg list

[Stephen Hemminger <stephen@networkplumber.org>]

On Mon,  1 Oct 2018 13:56:09 +0100
Anatoly Burakov <anatoly.burakov@intel.com> wrote:

> diff --git a/lib/librte_eal/common/include/rte_eal_memconfig.h b/lib/librte_eal/common/include/rte_eal_memconfig.h
> index aff0688dd..1d8b0a6fe 100644
> --- a/lib/librte_eal/common/include/rte_eal_memconfig.h
> +++ b/lib/librte_eal/common/include/rte_eal_memconfig.h
> @@ -30,6 +30,7 @@ struct rte_memseg_list {
>  		uint64_t addr_64;
>  		/**< Makes sure addr is always 64-bits */
>  	};
> +	size_t len; /**< Length of memory area covered by this memseg list. */
>  	int socket_id; /**< Socket ID for all memsegs in this list. */
>  	uint64_t page_sz; /**< Page size for all memsegs in this list. */
>  	volatile uint32_t version; /**< version number for multiprocess sync. */

If you are going to break ABI, why not try and rearrange to eliminate holes:

Output of pahole (on x86 64 bit):

struct rte_memseg_list {
	union {
		void *             base_va;              /*     0     8 */
		uint64_t           addr_64;              /*     0     8 */
	};                                               /*     0     8 */
	size_t                     len;                  /*     8     8 */
	int                        socket_id;            /*    16     4 */

	/* XXX 4 bytes hole, try to pack */

	uint64_t                   page_sz;              /*    24     8 */
	volatile uint32_t          version;              /*    32     4 */

	/* XXX 4 bytes hole, try to pack */

	struct rte_fbarray         memseg_arr;           /*    40    96 */

	/* XXX last struct has 4 bytes of padding */

	/* size: 136, cachelines: 3, members: 6 */
	/* sum members: 128, holes: 2, sum holes: 8 */
	/* paddings: 1, sum paddings: 4 */
	/* last cacheline: 8 bytes */
};