[PATCH v2] mbuf: replace GCC marker extension with C11 anonymous unions

[Tyler Retzlaff]

On Tue, Feb 13, 2024 at 05:58:21PM +0100, Morten Brørup wrote:
> > From: Tyler Retzlaff [mailto:roretzla at linux.microsoft.com]
> > Sent: Tuesday, 13 February 2024 07.46
> > 
> > Replace the use of RTE_MARKER<x> with C11 anonymous unions to improve
> > code portability between toolchains.
> 
> How about combining the cacheline 0 marker and padding, like this:

this seems like a good suggestion i will evaluate it. at least it gets
rid of all the extra nesting if there are no unforseen problems.

> 
>  struct rte_mbuf {
> -	RTE_MARKER cacheline0;
> +	union {
> +		char cacheline0[RTE_CACHE_LINE_MIN_SIZE];
>  
> +		struct {
> -	void *buf_addr;           /**< Virtual address of segment buffer. */
> +			void *buf_addr; /**< Virtual address of segment buffer. */
>  #if RTE_IOVA_IN_MBUF
> 
> 
> You could do the same with the cacheline1 marker:

yeah, i wondered if i should. i'll do it since it does seem more
consistent to just pad out both cachelines explicitly instead of just
doing all but the last.

we probably don't need to align struct rte_mbuf type if we do since it
will cause it to be naturally aligned to RTE_CACHE_LINE_MIN_SIZE.

> 
> 	/* second cache line - fields only used in slow path or on TX */
> -	RTE_MARKER cacheline1 __rte_cache_min_aligned;
> +	union {
> +		char cacheline1[RTE_CACHE_LINE_MIN_SIZE];
>  
> +		struct {
>  #if RTE_IOVA_IN_MBUF
> -	/**
> -	 * Next segment of scattered packet. Must be NULL in the last
> -	 * segment or in case of non-segmented packet.
> -	 */
> -	struct rte_mbuf *next;
> +			/**
> +			 * Next segment of scattered packet. Must be NULL in the last
> +			 * segment or in case of non-segmented packet.
> +			 */
> +			struct rte_mbuf *next;
>  #else
> 
> 
> It also avoids the weird union between cacheline0 and buf_addr at the beginning of the structure, and between cacheline1 and next/dynfield2 at the beginning of the second cache line.
> 
> And then you can omit the pad_cacheline0 array at the end of the first part of the structure.
> 
> 
> BTW: char is a weaker type than uint8_t - i.e. it is easier to cast to another type.
> It might be a personal preference, but I would use char instead of uint8_t for the padding array.

noted, i'll make the change.

thanks!