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@gmail.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);
 
 		xdp_convert_frame_to_buff(xdpf, &xdp);
 
-- 
2.20.1

From: Ujwal Kundur <ujwal.kundur@gmail.com>
Date: Mon,  9 Jun 2025 23:08:52 +0530

> 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@gmail.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);

I won't repeat what the folks above already told you.

I'll just add that you may want to take a look at Lorenzo's series[0].
Rx queue index is sorta HW hint, so it shouldn't be a problem to add the
corresponding field to xdp_rx_meta.
Then, you can expand cpumap's code to try reading that HW meta if present.

>  
>  		xdp_convert_frame_to_buff(xdpf, &xdp);

[0]
https://lore.kernel.org/bpf/174897275458.1677018.15827867832940584671.stgit@firesoul

Thanks,
Olek

> This looks wrong...
> I think this will always return index 0

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

I didn't realize that would always return 0, sorry I should've tried
to understand that statement better.

> I'll just add that you may want to take a look at Lorenzo's series[0].
> Rx queue index is sorta HW hint, so it shouldn't be a problem to add the
> corresponding field to xdp_rx_meta.
> Then, you can expand cpumap's code to try reading that HW meta if present.

Thank you! I also tried to work backwards to figure out how the
queue_index would be used if present in xdp_rxq_info but that wasn't
immediately apparent to me.
I'm keen on learning/contributing to the BPF part of the network stack
and this seemed like a good first patch to take up -- I'll understand
this better and try again.

Thanks for your time,
Ujwal

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

>> This looks wrong...
>> I think this will always return index 0
>
>> So passing dev->_rx to that function will always return 0; which is what
>> the field is already initialised to...
>
> I didn't realize that would always return 0, sorry I should've tried
> to understand that statement better.
>
>> I'll just add that you may want to take a look at Lorenzo's series[0].
>> Rx queue index is sorta HW hint, so it shouldn't be a problem to add the
>> corresponding field to xdp_rx_meta.
>> Then, you can expand cpumap's code to try reading that HW meta if present.
>
> Thank you! I also tried to work backwards to figure out how the
> queue_index would be used if present in xdp_rxq_info but that wasn't
> immediately apparent to me.
> I'm keen on learning/contributing to the BPF part of the network stack
> and this seemed like a good first patch to take up -- I'll understand
> this better and try again.

Sounds good! Don't be discouraged by having to try again, that's
perfectly normal :)

-Toke

Ujwal Kundur <ujwal.kundur@gmail.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@gmail.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

On 09/06/2025 19.38, Ujwal Kundur wrote:
> 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@gmail.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 looks wrong...
I think this will always return index 0

--Jesper