Re: include/soc/fsl/qman.h:383:4: warning: field context_a within 'struct qm_fqd' is less aligned than 'union (unnamed union at include/soc/fsl/qman.h:365:2)' and is usually due to 'struct qm_fqd' being packed, which can lead to unaligned accesses

From: Breno Leitao
Date: Wed Apr 09 2025 - 07:20:56 EST

On Wed, Apr 09, 2025 at 04:55:16PM +0800, kernel test robot wrote:
> >> include/soc/fsl/qman.h:383:4: warning: field context_a within 'struct qm_fqd' is less aligned than 'union (unnamed union at include/soc/fsl/qman.h:365:2)' and is usually due to 'struct qm_fqd' being packed, which can lead to unaligned accesses [-Wunaligned-access]
> } context_a;

<snip>

> c535e923bb97a4 Claudiu Manoil 2016-09-22 346
> c535e923bb97a4 Claudiu Manoil 2016-09-22 347 struct qm_fqd {
> c535e923bb97a4 Claudiu Manoil 2016-09-22 348 /* _res[6-7], orprws[3-5], oa[2], olws[0-1] */
> c535e923bb97a4 Claudiu Manoil 2016-09-22 349 u8 orpc;
> c535e923bb97a4 Claudiu Manoil 2016-09-22 350 u8 cgid;
> c535e923bb97a4 Claudiu Manoil 2016-09-22 351 __be16 fq_ctrl; /* See QM_FQCTRL_<...> */
> c535e923bb97a4 Claudiu Manoil 2016-09-22 352 __be16 dest_wq; /* channel[3-15], wq[0-2] */
> c535e923bb97a4 Claudiu Manoil 2016-09-22 353 __be16 ics_cred; /* 15-bit */
> c535e923bb97a4 Claudiu Manoil 2016-09-22 354 /*
> c535e923bb97a4 Claudiu Manoil 2016-09-22 355 * For "Initialize Frame Queue" commands, the write-enable mask
> c535e923bb97a4 Claudiu Manoil 2016-09-22 356 * determines whether 'td' or 'oac_init' is observed. For query
> c535e923bb97a4 Claudiu Manoil 2016-09-22 357 * commands, this field is always 'td', and 'oac_query' (below) reflects
> c535e923bb97a4 Claudiu Manoil 2016-09-22 358 * the Overhead ACcounting values.
> c535e923bb97a4 Claudiu Manoil 2016-09-22 359 */
> c535e923bb97a4 Claudiu Manoil 2016-09-22 360 union {
> c535e923bb97a4 Claudiu Manoil 2016-09-22 361 __be16 td; /* "Taildrop": _res[13-15], mant[5-12], exp[0-4] */
> c535e923bb97a4 Claudiu Manoil 2016-09-22 362 struct qm_fqd_oac oac_init;
> c535e923bb97a4 Claudiu Manoil 2016-09-22 363 };
> c535e923bb97a4 Claudiu Manoil 2016-09-22 364 __be32 context_b;
> c535e923bb97a4 Claudiu Manoil 2016-09-22 365 union {
> c535e923bb97a4 Claudiu Manoil 2016-09-22 366 /* Treat it as 64-bit opaque */
> c535e923bb97a4 Claudiu Manoil 2016-09-22 367 __be64 opaque;
> c535e923bb97a4 Claudiu Manoil 2016-09-22 368 struct {
> c535e923bb97a4 Claudiu Manoil 2016-09-22 369 __be32 hi;
> c535e923bb97a4 Claudiu Manoil 2016-09-22 370 __be32 lo;
> c535e923bb97a4 Claudiu Manoil 2016-09-22 371 };
> c535e923bb97a4 Claudiu Manoil 2016-09-22 372 /* Treat it as s/w portal stashing config */
> c535e923bb97a4 Claudiu Manoil 2016-09-22 373 /* see "FQD Context_A field used for [...]" */
> c535e923bb97a4 Claudiu Manoil 2016-09-22 374 struct {
> c535e923bb97a4 Claudiu Manoil 2016-09-22 375 struct qm_fqd_stashing stashing;
> c535e923bb97a4 Claudiu Manoil 2016-09-22 376 /*
> c535e923bb97a4 Claudiu Manoil 2016-09-22 377 * 48-bit address of FQ context to
> c535e923bb97a4 Claudiu Manoil 2016-09-22 378 * stash, must be cacheline-aligned
> c535e923bb97a4 Claudiu Manoil 2016-09-22 379 */

Looking at pahole, I am not sure this is caacheline-aligned, as
suggested above.

# pahole -C qm_fqd
struct qm_fqd {
u8 orpc; /* 0 1 */
u8 cgid; /* 1 1 */
__be16 fq_ctrl; /* 2 2 */
__be16 dest_wq; /* 4 2 */
__be16 ics_cred; /* 6 2 */
union {
__be16 td; /* 8 2 */
struct qm_fqd_oac oac_init; /* 8 2 */
}; /* 8 2 */
__be32 context_b; /* 10 4 */
union {
__be64 opaque; /* 14 8 */
struct {
__be32 hi; /* 14 4 */
__be32 lo; /* 18 4 */
}; /* 14 8 */
struct {
struct qm_fqd_stashing stashing; /* 14 2 */
__be16 context_hi; /* 16 2 */
__be32 context_lo; /* 18 4 */
}; /* 14 8 */
} context_a; /* 14 8 */
struct qm_fqd_oac oac_query; /* 22 2 */

/* size: 24, cachelines: 1, members: 9 */
/* last cacheline: 24 bytes */
} __attribute__((__packed__));


> c535e923bb97a4 Claudiu Manoil 2016-09-22 380 __be16 context_hi;
> c535e923bb97a4 Claudiu Manoil 2016-09-22 381 __be32 context_lo;
> c535e923bb97a4 Claudiu Manoil 2016-09-22 382 } __packed;

I am wondering if we should use __attribute__((aligned(8))) instead of
__packed, according to the document above.