Re: [PATCH v13 bpf-next 02/18] xdp: introduce flags field in xdp_buff/xdp_frame

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

(Minor changes requested below)

On 07/09/2021 14.35, Lorenzo Bianconi wrote:
> Introduce flags field in xdp_frame and xdp_buffer data structures
> to define additional buffer features. At the moment the only
> supported buffer feature is multi-buffer bit (mb). Multi-buffer bit
> is used to specify if this is a linear buffer (mb = 0) or a multi-buffer
> frame (mb = 1). In the latter case the driver is expected to initialize
> the skb_shared_info structure at the end of the first buffer to link
> together subsequent buffers belonging to the same frame.
> 
> Acked-by: John Fastabend <john.fastabend@gmail.com>
> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> ---
>   include/net/xdp.h | 29 +++++++++++++++++++++++++++++
>   1 file changed, 29 insertions(+)
> 
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index ad5b02dcb6f4..ed5ea784fd45 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -66,6 +66,10 @@ struct xdp_txq_info {
>   	struct net_device *dev;
>   };
>   
> +enum xdp_buff_flags {
> +	XDP_FLAGS_MULTI_BUFF	= BIT(0), /* non-linear xdp buff */
> +};
> +
>   struct xdp_buff {
>   	void *data;
>   	void *data_end;
> @@ -74,13 +78,30 @@ struct xdp_buff {
>   	struct xdp_rxq_info *rxq;
>   	struct xdp_txq_info *txq;
>   	u32 frame_sz; /* frame size to deduce data_hard_end/reserved tailroom*/
> +	u16 flags; /* supported values defined in xdp_flags */
                                                   ^^^^^^^^^
Variable/enum is named "xdp_buff_flags", but comment says "xdp_flags".

I think we should change flags to use u32, because xdp_buff already 
contain 4 byte padding. (pahole output provided as help below)

>   };
>   
> +static __always_inline bool xdp_buff_is_mb(struct xdp_buff *xdp)
> +{
> +	return !!(xdp->flags & XDP_FLAGS_MULTI_BUFF);
> +}
> +
> +static __always_inline void xdp_buff_set_mb(struct xdp_buff *xdp)
> +{
> +	xdp->flags |= XDP_FLAGS_MULTI_BUFF;
> +}
> +
> +static __always_inline void xdp_buff_clear_mb(struct xdp_buff *xdp)
> +{
> +	xdp->flags &= ~XDP_FLAGS_MULTI_BUFF;
> +}
> +
>   static __always_inline void
>   xdp_init_buff(struct xdp_buff *xdp, u32 frame_sz, struct xdp_rxq_info *rxq)
>   {
>   	xdp->frame_sz = frame_sz;
>   	xdp->rxq = rxq;
> +	xdp->flags = 0;
>   }
>   
>   static __always_inline void
> @@ -122,8 +143,14 @@ struct xdp_frame {
>   	 */
>   	struct xdp_mem_info mem;
>   	struct net_device *dev_rx; /* used by cpumap */
> +	u16 flags; /* supported values defined in xdp_flags */
                                                   ^^^^^^^^^
Variable/enum is named "xdp_buff_flags", but comment says "xdp_flags".

Here (for xdp_frame) I also think we should change flags to u32, because 
adding this u16 cause extra padding anyhow. (pahole output provided as 
help below).


>   };
>   
> +static __always_inline bool xdp_frame_is_mb(struct xdp_frame *frame)
> +{
> +	return !!(frame->flags & XDP_FLAGS_MULTI_BUFF);
> +}
> +
>   #define XDP_BULK_QUEUE_SIZE	16
>   struct xdp_frame_bulk {
>   	int count;
> @@ -180,6 +207,7 @@ void xdp_convert_frame_to_buff(struct xdp_frame *frame, struct xdp_buff *xdp)
>   	xdp->data_end = frame->data + frame->len;
>   	xdp->data_meta = frame->data - frame->metasize;
>   	xdp->frame_sz = frame->frame_sz;
> +	xdp->flags = frame->flags;
>   }
>   
>   static inline
> @@ -206,6 +234,7 @@ int xdp_update_frame_from_buff(struct xdp_buff *xdp,
>   	xdp_frame->headroom = headroom - sizeof(*xdp_frame);
>   	xdp_frame->metasize = metasize;
>   	xdp_frame->frame_sz = xdp->frame_sz;
> +	xdp_frame->flags = xdp->flags;
>   
>   	return 0;
>   }
> 



Details below... no need to read any further

Investigating struct xdp_frame with pahole:

$ pahole -C xdp_frame net/core/xdp.o
struct xdp_frame {
	void *                     data;             /*     0     8 */
	u16                        len;              /*     8     2 */
	u16                        headroom;         /*    10     2 */
	u32                        metasize:8;       /*    12: 0  4 */
	u32                        frame_sz:24;      /*    12: 8  4 */
	struct xdp_mem_info        mem;              /*    16     8 */
	struct net_device *        dev_rx;           /*    24     8 */

	/* size: 32, cachelines: 1, members: 7 */
	/* last cacheline: 32 bytes */
};


  pahole -C xdp_frame net/core/xdp.o
struct xdp_frame {
	void *                     data;             /*     0     8 */
	u16                        len;              /*     8     2 */
	u16                        headroom;         /*    10     2 */
	u32                        metasize:8;       /*    12: 0  4 */
	u32                        frame_sz:24;      /*    12: 8  4 */
	struct xdp_mem_info        mem;              /*    16     8 */
	struct net_device *        dev_rx;           /*    24     8 */
	u16                        flags;            /*    32     2 */

	/* size: 40, cachelines: 1, members: 8 */
	/* padding: 6 */
	/* last cacheline: 40 bytes */
};


$ pahole -C xdp_frame net/core/xdp.o
struct xdp_frame {
	void *                     data;             /*     0     8 */
	u16                        len;              /*     8     2 */
	u16                        headroom;         /*    10     2 */
	u32                        metasize:8;       /*    12: 0  4 */
	u32                        frame_sz:24;      /*    12: 8  4 */
	struct xdp_mem_info        mem;              /*    16     8 */
	struct net_device *        dev_rx;           /*    24     8 */
	u32                        flags;            /*    32     4 */

	/* size: 40, cachelines: 1, members: 8 */
	/* padding: 4 */
	/* last cacheline: 40 bytes */
};


Details for struct xdp_buff, it already contains 4 bytes padding.

$ pahole -C xdp_buff net/core/xdp.o
struct xdp_buff {
	void *                     data;             /*     0     8 */
	void *                     data_end;         /*     8     8 */
	void *                     data_meta;        /*    16     8 */
	void *                     data_hard_start;  /*    24     8 */
	struct xdp_rxq_info *      rxq;              /*    32     8 */
	struct xdp_txq_info *      txq;              /*    40     8 */
	u32                        frame_sz;         /*    48     4 */
	u16                        flags;            /*    52     2 */

	/* size: 56, cachelines: 1, members: 8 */
	/* padding: 2 */
	/* last cacheline: 56 bytes */
};