Re: [PATCH v2 net 2/8] xsk: respect tailroom for ZC setups

[Stanislav Fomichev <stfomichev@gmail.com>]

On 03/19, Maciej Fijalkowski wrote:
> Multi-buffer XDP stores information about frags in skb_shared_info that
> sits at the tailroom of a packet. The storage space is reserved via
> xdp_data_hard_end():
> 
> 	((xdp)->data_hard_start + (xdp)->frame_sz -	\
> 	 SKB_DATA_ALIGN(sizeof(struct skb_shared_info)))
> 
> and then we refer to it via macro below:
> 
> static inline struct skb_shared_info *
> xdp_get_shared_info_from_buff(const struct xdp_buff *xdp)
> {
>         return (struct skb_shared_info *)xdp_data_hard_end(xdp);
> }
> 
> Currently we do not respect this tailroom space in multi-buffer AF_XDP
> ZC scenario. To address this, introduce xsk_pool_get_tailroom() and use
> it within xsk_pool_get_rx_frame_size() which is used in ZC drivers to
> configure length of HW Rx buffer.
> 
> xsk_pool_get_tailroom() is only reserving necessary space when pool is
> zc and underlying netdev supports zc multi-buffer. Rely on pool->dev
> state when configuring tailroom. xsk_pool_get_rx_frame_size() inside
> ndo_bpf is usually called when bringing up queues and before xsk's dma
> mappings have been configured, which makes it valid to rely on
> pool->dev.
> 
> Fixes: 24ea50127ecf ("xsk: support mbuf on ZC RX")
> Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>

Acked-by: Stanislav Fomichev <sdf@fomichev.me>