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

[Lorenzo Bianconi <lorenzo.bianconi@redhat.com>]

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> (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".

ack, I will fix it in v14

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

ack, I will fix it in v14
> 
> >   };
> > +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).

ack, I will fix it in v14
> 
> 
> >   };
> > +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 */
> };

ack, right.

Regards,
Lorenzo

> 

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