Re: [PATCH net] net/mlx5e: Precompute xdpsq assignments for mlx5e_xdp_xmit()

[Tariq Toukan <ttoukan.linux@gmail.com>]

On 23/02/2026 2:05, Finn Dayton wrote:
> mlx5e_xdp_xmit() selects an XDP SQ (Send Queue) using smp_processor_id()
> (CPU ID). When doing XDP_REDIRECT from a CPU whose ID is
>> = priv->channels.num, mlx5e_xdp_xmit() returns -ENXIO and the
> redirect fails.
> 
> Previous discussion proposed using modulo in mlx5e_xdp_xmit() to map
> CPU IDs into the channel range, but modulo/division is too costly in
> the hot path.
> 

That discussion reached to an agreement of using a while loop with 
subtraction. It's expected to be fast, and optimizes the common case.

> Instead, this solution precomputes per-cpu priv->xdpsq assignments when
> channels are (re)configured and does a single lookup in  mlx5e_xdp_xmit().
> 
> Because multiple CPUs map to the same xdpsq when CPU count exceeds
> channel count, serialize xdp_xmit on the ring with xdp_tx_lock.
> 

What's the advantage of this solution over the one we already agreed to?

> Fixes: 58b99ee3e3eb ("net/mlx5e: Add support for XDP_REDIRECT in device-out side")
> Link: https://lore.kernel.org/netdev/20251031231038.1092673-1-zijianzhang@bytedance.com/
> Link: https://lore.kernel.org/netdev/44f69955-b566-4fb1-904d-f551046ff2d4@gmail.com
> Cc: stable@vger.kernel.org # 6.12+
> Signed-off-by: Finn Dayton <finnius.dayton@spacex.com>
> ---
> Testing:
>   - XDP forwarding / XDP_REDIRECT verified with both low CPU ids and
>     CPU ids > than number of send queues.
>   - No -ENXIO observed, successful forwarding.
> 
>   drivers/net/ethernet/mellanox/mlx5/core/en.h  |  4 +++
>   .../net/ethernet/mellanox/mlx5/core/en/xdp.c  | 16 +++++++----
>   .../net/ethernet/mellanox/mlx5/core/en_main.c | 28 +++++++++++++++++++
>   3 files changed, 43 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en.h b/drivers/net/ethernet/mellanox/mlx5/core/en.h
> index ea2cd1f5d1d0..387954201640 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en.h
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en.h
> @@ -519,6 +519,8 @@ struct mlx5e_xdpsq {
>   	/* control path */
>   	struct mlx5_wq_ctrl        wq_ctrl;
>   	struct mlx5e_channel      *channel;
> +	/* serialize writes by multiple CPUs to this send queue */
> +	spinlock_t xdp_tx_lock;
>   } ____cacheline_aligned_in_smp;
>   
>   struct mlx5e_xdp_buff {
> @@ -909,6 +911,8 @@ struct mlx5e_priv {
>   	struct mlx5e_rq            drop_rq;
>   
>   	struct mlx5e_channels      channels;
> +	/* selects the xdpsq during mlx5e_xdp_xmit() */
> +	int __percpu              *send_queue_idx_ptr;
>   	struct mlx5e_rx_res       *rx_res;
>   	u32                       *tx_rates;
>   
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c b/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
> index 80f9fc10877a..2dd44ad873a1 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
> @@ -845,7 +845,7 @@ int mlx5e_xdp_xmit(struct net_device *dev, int n, struct xdp_frame **frames,
>   	struct mlx5e_priv *priv = netdev_priv(dev);
>   	struct mlx5e_xdpsq *sq;
>   	int nxmit = 0;
> -	int sq_num;
> +	int send_queue_idx = 0;
>   	int i;
>   
>   	/* this flag is sufficient, no need to test internal sq state */
> @@ -855,13 +855,19 @@ int mlx5e_xdp_xmit(struct net_device *dev, int n, struct xdp_frame **frames,
>   	if (unlikely(flags & ~XDP_XMIT_FLAGS_MASK))
>   		return -EINVAL;
>   
> -	sq_num = smp_processor_id();
>   
> -	if (unlikely(sq_num >= priv->channels.num))
> +	if (unlikely(!priv->send_queue_idx_ptr))
>   		return -ENXIO;
>   
> -	sq = priv->channels.c[sq_num]->xdpsq;
> +	send_queue_idx = *this_cpu_ptr(priv->send_queue_idx_ptr);
> +	if (unlikely(send_queue_idx >= priv->channels.num || send_queue_idx < 0))
> +		return -ENXIO;
>   
> +	sq = priv->channels.c[send_queue_idx]->xdpsq;
> +	/* The number of queues configured on a netdev may be smaller than the
> +	 * CPU pool, so two CPUs might map to this queue. We must serialize writes.
> +	 */
> +	spin_lock(&sq->xdp_tx_lock);
>   	for (i = 0; i < n; i++) {
>   		struct mlx5e_xmit_data_frags xdptxdf = {};
>   		struct xdp_frame *xdpf = frames[i];
> @@ -941,7 +947,7 @@ int mlx5e_xdp_xmit(struct net_device *dev, int n, struct xdp_frame **frames,
>   
>   	if (flags & XDP_XMIT_FLUSH)
>   		mlx5e_xmit_xdp_doorbell(sq);
> -
> +	spin_unlock(&sq->xdp_tx_lock);
>   	return nxmit;
>   }
>   
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> index 7eb691c2a1bd..adef35d06b89 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> @@ -1492,6 +1492,7 @@ static int mlx5e_alloc_xdpsq(struct mlx5e_channel *c,
>   	sq->pdev      = c->pdev;
>   	sq->mkey_be   = c->mkey_be;
>   	sq->channel   = c;
> +	spin_lock_init(&sq->xdp_tx_lock);
>   	sq->uar_map   = c->bfreg->map;
>   	sq->min_inline_mode = params->tx_min_inline_mode;
>   	sq->hw_mtu    = MLX5E_SW2HW_MTU(params, params->sw_mtu) - ETH_FCS_LEN;
> @@ -3283,10 +3284,30 @@ static void mlx5e_build_txq_maps(struct mlx5e_priv *priv)
>   	smp_wmb();
>   }
>   
> +static void build_priv_to_xdpsq_associations(struct mlx5e_priv *priv)
> +{
> +	/*
> +	 * Build the mapping from CPU to XDP send queue index for priv.
> +	 * This is used by mlx5e_xdp_xmit() to determine which xdpsq (send queue)
> +	 * should handle the xdptx data, based on the CPU running mlx5e_xdp_xmit()
> +	 * and the target priv (netdev).
> +	 */
> +	int send_queue_idx, cpu;
> +
> +	if (unlikely(priv->channels.num == 0))
> +		return;
> +
> +	for_each_possible_cpu(cpu) {
> +		send_queue_idx = cpu % priv->channels.num;
> +		*per_cpu_ptr(priv->send_queue_idx_ptr, cpu) = send_queue_idx;
> +	}
> +}
> +
>   void mlx5e_activate_priv_channels(struct mlx5e_priv *priv)
>   {
>   	mlx5e_build_txq_maps(priv);
>   	mlx5e_activate_channels(priv, &priv->channels);
> +	build_priv_to_xdpsq_associations(priv);
>   	mlx5e_xdp_tx_enable(priv);
>   
>   	/* dev_watchdog() wants all TX queues to be started when the carrier is
> @@ -6263,8 +6284,14 @@ int mlx5e_priv_init(struct mlx5e_priv *priv,
>   	if (!priv->fec_ranges)
>   		goto err_free_channel_stats;
>   
> +	priv->send_queue_idx_ptr = alloc_percpu(int);
> +	if (!priv->send_queue_idx_ptr)
> +		goto err_free_fec_ranges;
> +
>   	return 0;
>   
> +err_free_fec_ranges:
> +	kfree(priv->fec_ranges);
>   err_free_channel_stats:
>   	kfree(priv->channel_stats);
>   err_free_tx_rates:
> @@ -6295,6 +6322,7 @@ void mlx5e_priv_cleanup(struct mlx5e_priv *priv)
>   	for (i = 0; i < priv->stats_nch; i++)
>   		kvfree(priv->channel_stats[i]);
>   	kfree(priv->channel_stats);
> +	free_percpu(priv->send_queue_idx_ptr);
>   	kfree(priv->tx_rates);
>   	kfree(priv->txq2sq_stats);
>   	kfree(priv->txq2sq);