Re: [PATCH v2] graph: expose node context as pointers

[Tyler Retzlaff <roretzla@linux.microsoft.com>]

On Fri, Mar 22, 2024 at 05:31:31PM +0100, Robin Jarry wrote:
> In some cases, the node context data is used to store two pointers
> because the data is larger than the reserved 16 bytes. Having to define
> intermediate structures just to be able to cast is tedious. Add two
> pointers that take the same space than ctx.
> 
> Signed-off-by: Robin Jarry <rjarry@redhat.com>
> ---
> 
> Notes:
>     v2:
>     
>     * Added __extension__ (not sure where it is needed, I don't have access to windows).

i can answer this!

windows toolchains will only require __extension__ qualification on use
of statement expressions, so msvc won't require any use of __extension__
in this patch.

as a general rule of thumb __extension__ is something you may choose to
use for any gcc compiled code that is an extension to standard C and you
intend to use the -pedantic flag (i.e. -std=c11 && -pedantic used together)

i've commented inline below.

>     * It still fails the header check for C++. It seems not possible to align an unnamed union...
>       Tyler, do you have an idea about how to fix that?

yes, see below what i suspect you want.

>     * Added static_assert to ensure the anonymous union is not larger than RTE_NODE_CTX_SZ.
> 
>  lib/graph/rte_graph_worker_common.h | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/graph/rte_graph_worker_common.h b/lib/graph/rte_graph_worker_common.h
> index 36d864e2c14e..a60c2bc3f0c3 100644
> --- a/lib/graph/rte_graph_worker_common.h
> +++ b/lib/graph/rte_graph_worker_common.h
> @@ -112,7 +112,14 @@ struct __rte_cache_aligned rte_node {
>  	};
>  	/* Fast path area  */
>  #define RTE_NODE_CTX_SZ 16
> -	alignas(RTE_CACHE_LINE_SIZE) uint8_t ctx[RTE_NODE_CTX_SZ]; /**< Node Context. */
> +	__extension__ alignas(RTE_CACHE_LINE_SIZE) union {

__extension__ should not be on the anonymous union (since they are standard C11).

anonymous union declaration is actually a type with no name and then a data
field of that type so __rte_aligned is most likely what you want, since
you're using RTE_CACHE_LINE_SIZE we can use __rte_cache_aligned.

union __rte_cache_aligned {
   ... your union fields ...
};

and i think checkpatches still gives a warning unrelated to alignment
for this but it can be safely ignored. it's the warning about alignment
that we care about and should be fixed.

> +		uint8_t ctx[RTE_NODE_CTX_SZ];
> +		/* Convenience aliases to store pointers without complex casting. */
> +		__extension__ struct {

this is correct/recommended since anonymous structs aren't standard,
with the __extension__ -pedantic won't emit a warning (our intention).

> +			void *ctx_ptr;
> +			void *ctx_ptr2;
> +		};
> +	}; /**< Node Context. */
>  	uint16_t size;		/**< Total number of objects available. */
>  	uint16_t idx;		/**< Number of objects used. */
>  	rte_graph_off_t off;	/**< Offset of node in the graph reel. */
> @@ -130,6 +137,9 @@ struct __rte_cache_aligned rte_node {
>  	alignas(RTE_CACHE_LINE_MIN_SIZE) struct rte_node *nodes[]; /**< Next nodes. */
>  };
>  
> +static_assert(offsetof(struct rte_node, size) - offsetof(struct rte_node, ctx) == RTE_NODE_CTX_SZ,
> +	"The node context anonymous union cannot be larger than RTE_NODE_CTX_SZ");
> +

you should include directly include <stddef.h> in this file for use of offsetof.
you should include directly include <assert.h> in this file for use of the static_assert.

hope this helps!

ty

>  /**
>   * @internal
>   *
> -- 
> 2.44.0