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

[Breno Leitao <leitao@debian.org>]

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.