linux-kernel - Re: [PATCH] bpf: cpumap: report Rx queue index to xdp_rxq

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Message-ID: <874iwnerfi.fsf@toke.dk>
Date: Tue, 10 Jun 2025 14:01:37 +0200
From: Toke Høiland-Jørgensen <toke@...nel.org>
To: Ujwal Kundur <ujwal.kundur@...il.com>, ast@...nel.org,
 daniel@...earbox.net, davem@...emloft.net, kuba@...nel.org,
 hawk@...nel.org, john.fastabend@...il.com, andrii@...nel.org,
 martin.lau@...ux.dev, eddyz87@...il.com, song@...nel.org,
 yonghong.song@...ux.dev, kpsingh@...nel.org, sdf@...ichev.me,
 aoluo@...gle.com, jolsa@...nel.org
Cc: netdev@...r.kernel.org, bpf@...r.kernel.org,
 linux-kernel@...r.kernel.org, Ujwal Kundur <ujwal.kundur@...il.com>
Subject: Re: [PATCH] bpf: cpumap: report Rx queue index to xdp_rxq_info

Ujwal Kundur <ujwal.kundur@...il.com> writes:

> Refer to the Rx queue using a XDP frame's attached netdev and ascertain
> the queue index from it.
>
> Signed-off-by: Ujwal Kundur <ujwal.kundur@...il.com>
> ---
>  kernel/bpf/cpumap.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/cpumap.c b/kernel/bpf/cpumap.c
> index 67e8a2fc1a99..8230292deac1 100644
> --- a/kernel/bpf/cpumap.c
> +++ b/kernel/bpf/cpumap.c
> @@ -34,6 +34,7 @@
>  #include <linux/btf_ids.h>
>  
>  #include <linux/netdevice.h>
> +#include <net/netdev_rx_queue.h>
>  #include <net/gro.h>
>  
>  /* General idea: XDP packets getting XDP redirected to another CPU,
> @@ -196,7 +197,7 @@ static int cpu_map_bpf_prog_run_xdp(struct bpf_cpu_map_entry *rcpu,
>  
>  		rxq.dev = xdpf->dev_rx;
>  		rxq.mem.type = xdpf->mem_type;
> -		/* TODO: report queue_index to xdp_rxq_info */
> +		rxq.queue_index = get_netdev_rx_queue_index(xdpf->dev_rx->_rx);

This is pretty nonsensical; the definition of the function you're
calling is this:

static inline unsigned int
get_netdev_rx_queue_index(struct netdev_rx_queue *queue)
{
	struct net_device *dev = queue->dev;
	int index = queue - dev->_rx;

	BUG_ON(index >= dev->num_rx_queues);
	return index;
}

So passing dev->_rx to that function will always return 0; which is what
the field is already initialised to...

-Toke