[net-next,3/3] net/mlx4_en: RX, Add a prefetch command for small L1_CACHE_BYTES

A single cacheline might not contain the packet header for
small L1_CACHE_BYTES values.
Use net_prefetch() as it issues an additional prefetch
in this case.

Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Reviewed-by: Saeed Mahameed <saeedm@mellanox.com>
---
 drivers/net/ethernet/mellanox/mlx4/en_rx.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

On 8/26/20 5:54 AM, Tariq Toukan wrote:
> A single cacheline might not contain the packet header for
> small L1_CACHE_BYTES values.
> Use net_prefetch() as it issues an additional prefetch
> in this case.
> 
> Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
> Reviewed-by: Saeed Mahameed <saeedm@mellanox.com>
> ---
>  drivers/net/ethernet/mellanox/mlx4/en_rx.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx4/en_rx.c b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> index b50c567ef508..99d7737e8ad6 100644
> --- a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> +++ b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> @@ -705,7 +705,7 @@ int mlx4_en_process_rx_cq(struct net_device *dev, struct mlx4_en_cq *cq, int bud
>  
>  		frags = ring->rx_info + (index << priv->log_rx_info);
>  		va = page_address(frags[0].page) + frags[0].page_offset;
> -		prefetchw(va);
> +		net_prefetchw(va);
>  		/*
>  		 * make sure we read the CQE after we read the ownership bit
>  		 */
> 

Why these cache lines would be written next ? Presumably we read the headers (pulled into skb->head)

Really using prefetch() for the about to be read packet is too late anyway for current cpus.

On Wed, 2020-08-26 at 07:55 -0700, Eric Dumazet wrote:
> 
> On 8/26/20 5:54 AM, Tariq Toukan wrote:
> > A single cacheline might not contain the packet header for
> > small L1_CACHE_BYTES values.
> > Use net_prefetch() as it issues an additional prefetch
> > in this case.
> > 
> > Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
> > Reviewed-by: Saeed Mahameed <saeedm@mellanox.com>
> > ---
> >  drivers/net/ethernet/mellanox/mlx4/en_rx.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> > b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> > index b50c567ef508..99d7737e8ad6 100644
> > --- a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> > +++ b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> > @@ -705,7 +705,7 @@ int mlx4_en_process_rx_cq(struct net_device
> > *dev, struct mlx4_en_cq *cq, int bud
> >  
> >  		frags = ring->rx_info + (index << priv->log_rx_info);
> >  		va = page_address(frags[0].page) +
> > frags[0].page_offset;
> > -		prefetchw(va);
> > +		net_prefetchw(va);
> >  		/*
> >  		 * make sure we read the CQE after we read the
> > ownership bit
> >  		 */
> > 
> 
> Why these cache lines would be written next ? Presumably we read the
> headers (pulled into skb->head)
> 

XDP

> Really using prefetch() for the about to be read packet is too late
> anyway for current cpus.
>