i40e: Fix XDP redirect failure when queue count < CPU count

[diegonix]

Summary

This patch fixes XDP redirect operations failing with -ENXIO when the number of configured network queues is less than the number of online CPUs.

Problem Description

The i40e_xdp_xmit() function uses smp_processor_id() directly as a queue index without proper bounds checking. In performance-optimized environments where users configure fewer queues than CPUs (e.g., 4 queues on a 6-CPU system), XDP redirect operations from higher-numbered CPUs fail with errno 6 (ENXIO).

Root Cause

• File: src/i40e_txrx.c
• Function: i40e_xdp_xmit()
• Issue: queue_index = smp_processor_id() can exceed vsi->num_queue_pairs
• Result: CPUs 4-5 accessing 4 queues (0-3) triggers bounds check failure

Solution

Implement modulo-based CPU-to-queue mapping to ensure queue index never exceeds available queues:

• Add i40e_xdp_queue_index() helper function using modulo operation
• Add configuration warning in ethtool when setting channels < online CPUs with XDP enabled
• Improve error messages for better diagnostics
• Clean up debug messages to reduce noise

Testing

Test Environment

• Hardware: Intel XL710 40GbE NIC (PCI ID: 0000:04:00.0)
• Firmware: fw 9.154.78653 api 1.15 nvm 9.54
• Driver: i40e v2.28.7 (patched)
• Kernel: 5.15.0-309.180.4.el9uek.x86_64 (Oracle Linux)
• System: 6-CPU server

Test Configuration

# Configure 4 queues on 6-CPU system
ethtool -L data0 combined 4
# Load XDP program for packet processing
# Result: XDP redirect from CPUs 4-5 previously failed

Additional Notes

• This fix was developed on v2.28.7 release
• Also tested in combination with pending [PR#7] without conflicts
• The fix is independent and can be merged without dependencies

Signed-off-by: Diego diegonix@gmail.com

[mw70830]

Wouldn't it be simpler to fix it like the following, as other drivers do?
queue_index = smp_processor_id() % vsi->num_queue_pairs;

For reference, here’s how the ixgbe driver handles it:

static inline int ixgbe_determine_xdp_q_idx(int cpu) 
{
    if (static_key_enabled(&ixgbe_xdp_locking_key))
        return cpu % IXGBE_MAX_XDP_QS;
    else
        return cpu;
}

static inline
struct ixgbe_ring *ixgbe_determine_xdp_ring(struct ixgbe_adapter *adapter)
{
    int index = ixgbe_determine_xdp_q_idx(smp_processor_id());

    return adapter->xdp_ring[index];
}

[diegonix]

I'm sorry, I was working with the i40e, I came across this problem and thought I would fix it right away, I didn't think about doing a degree in Intel network drivers before proposing something.