Re: [PATCH v3 12/18] malloc: fix mp message alignment

[Bruce Richardson <bruce.richardson@intel.com>]

On Tue, Jul 08, 2025 at 02:28:16PM +0200, David Marchand wrote:
> Content (param[]) of received multiprocess messages are aligned with
> a 4 bytes constraint.
> 
> Before patch:
> struct mp_msg_internal {
>  int type;                                                     /*   0     4 */
>  struct rte_mp_msg {
>   char name[64];                                               /*   4    64 */
>   /* --- cacheline 1 boundary (64 bytes) was 4 bytes ago --- */
>   int len_param;                                               /*  68     4 */
>   int num_fds;                                                 /*  72     4 */
>   /* typedef uint8_t -> __uint8_t */ unsigned char param[256]; /*  76   256 */
>   /* --- cacheline 5 boundary (320 bytes) was 12 bytes ago --- */
>   int fds[253];                                                /* 332  1012 */
>  } msg;                                                        /*   4  1340 */
> 
>  /* size: 1344, cachelines: 21, members: 2 */
> };
> 
> This results in many unaligned accesses for multiprocess malloc requests.
> 
> Examples:
> ../lib/eal/common/malloc_mp.c:308:32: runtime error:
> 	member access within misaligned address 0x7f7b35df4684 for type
> 	'const struct malloc_mp_req', which requires 8 byte alignment
> 
> ../lib/eal/common/malloc_mp.c:158:9: runtime error:
> 	member access within misaligned address 0x7f36a535bb5c for type
> 	'const struct malloc_mp_req', which requires 8 byte alignment
> 
> ../lib/eal/common/malloc_mp.c:171:8: runtime error:
> 	member access within misaligned address 0x7f4ba65f296c for type
> 	'struct malloc_mp_req', which requires 8 byte alignment
> 
> Signed-off-by: David Marchand <david.marchand@redhat.com>
> ---
>  lib/eal/common/eal_common_proc.c |  2 +-
>  lib/eal/common/malloc_mp.c       | 18 +++++++++---------
>  2 files changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/lib/eal/common/eal_common_proc.c b/lib/eal/common/eal_common_proc.c
> index 0dea787e38..3846c7178d 100644
> --- a/lib/eal/common/eal_common_proc.c
> +++ b/lib/eal/common/eal_common_proc.c
> @@ -62,7 +62,7 @@ enum mp_type {
>  
>  struct mp_msg_internal {
>  	int type;
> -	struct rte_mp_msg msg;
> +	alignas(8) struct rte_mp_msg msg;
>  };
>  
>  struct async_request_param {
> diff --git a/lib/eal/common/malloc_mp.c b/lib/eal/common/malloc_mp.c
> index 9765277f5d..000c7f6b47 100644
> --- a/lib/eal/common/malloc_mp.c
> +++ b/lib/eal/common/malloc_mp.c
> @@ -148,7 +148,7 @@ get_unique_id(void)
>  static int
>  handle_sync(const struct rte_mp_msg *msg, const void *peer)
>  {
> -	struct rte_mp_msg reply;
> +	alignas(8) struct rte_mp_msg reply;
>  	const struct malloc_mp_req *req =
>  			(const struct malloc_mp_req *)msg->param;

This patch seems to have a lot of these definitions with alignas added to
them. Would it be simpler just to put the alignas inside the rte_mp_msg
definition?

More specifically, if its the "uint8_t param" element that needs alignment,
how about changing that specific field to make it aligned?

/Bruce