[DPDK/ethdev Bug 1776] Segmentation fault encountered in MPRQ vectorized mode

[bugzilla@dpdk.org]

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https://bugs.dpdk.org/show_bug.cgi?id=1776

            Bug ID: 1776
           Summary: Segmentation fault encountered in MPRQ vectorized mode
           Product: DPDK
           Version: 22.11
          Hardware: x86
                OS: Linux
            Status: UNCONFIRMED
          Severity: critical
          Priority: Normal
         Component: ethdev
          Assignee: dev@dpdk.org
          Reporter: canary.overflow@gmail.com
  Target Milestone: ---

I have been encountering segmentation fault when running DPDK in MPRQ
vectorized mode. To reproduce the issue on testpmd, run with the following
parameters:

dpdk-testpmd -l 1-5 -n 4 -a
0000:1f:00.0,rxq_comp_en=1,rxq_pkt_pad_en=1,rxqs_min_mprq=1,mprq_en=1,mprq_log_stride_num=6,mprq_log_stride_size=9,mprq_max_memcpy_len=64,rx_vec_en=1
-- -i --rxd=8192 --max-pkt-len=9000 --rxq=1 --total-num-mbufs=16384
--mbuf-size=3000 --enable-drop-en –-enable-scatter

This segmentation fault goes away when I disable vectorization (rx_vec_en=0).
(Note that the segmentation fault does not occur in forward-mode=rxonly). The
segmentation fault also seems to happen with higher chances when there is a
rxnombuf.

The backtrace of the segmentation fault was:
#0  0x0000000001c34912 in __rte_pktmbuf_free_extbuf ()
#1  0x0000000001c36a10 in rte_pktmbuf_detach ()
#2  0x0000000001c4a9ec in rxq_copy_mprq_mbuf_v ()
#3  0x0000000001c4d63b in rxq_burst_mprq_v ()
#4  0x0000000001c4d7a7 in mlx5_rx_burst_mprq_vec ()
#5  0x000000000050be66 in rte_eth_rx_burst ()
#6  0x000000000050c53d in pkt_burst_io_forward ()
#7  0x00000000005427b4 in run_pkt_fwd_on_lcore ()
#8  0x000000000054289b in start_pkt_forward_on_core ()
#9  0x0000000000a473c9 in eal_thread_loop ()
#10 0x00007ffff60061ca in start_thread () from /lib64/libpthread.so.0
#11 0x00007ffff5c72e73 in clone () from /lib64/libc.so.6

*Note that the addresses may not be exact as I've added some log statements and
attempted fixes previously (they were commented out when I obtained this
backtrace).

Upon some investigation, I noticed that in DPDK’s source codes
drivers/net/mlx5/mlx5_rxtx_vec.c (function rxq_copy_mprq_mbuf_v()), there is a
possibility where the consumed stride exceeds the stride number (64 in this
case) which should not be happening. I'm suspecting that there's some CQE
misalignment here upon encountering rxnombuf.

rxq_copy_mprq_mbuf_v(...) {
    ...
    if(rxq->consumed_strd == strd_n) {   
        // replenish WQE
    }
    ...
    strd_cnt = (elts[i]->pkt_len / strd_sz) + 
               ((elts[i]->pkt_len % strd_sz) ? 1 : 0);

    rxq_code = mprq_buf_to_pkt(rxq, elts[i], elts[i]->pkt_len, buf,
rxq->consumed_strd, strd_cnt);
    rxq->consumed_strd += strd_cnt;       // encountering cases where
rxq->consumed_strd > strd_n
    ...
}

In addition, there were also cases in mprq_buf_to_pkt() where the allocated seg
address is exactly the same as the pkt (elts[i]) address passed in which should
not happen.

mprq_buf_to_pkt(...) {
    ...
    if(hdrm_overlap > 0) {   
        MLX5_ASSERT(rxq->strd_scatter_en);
        struct rte_mbuf *seg = rte_pktmbuf_alloc(rxq->mp);
        if (unlikely(seg == NULL)) return MLX5_RXQ_CODE_NOMBUF;
        SET_DATA_OFF(seg, 0);

        // added debug statement
        // saw instances where pkt = seg
        DRV_LOG(DEBUG, "pkt %p seg %p", (void *)pkt, (void *)seg);
        rte_memcpy(rte_pktmbuf_mtod(seg, void *), RTE_PTR_ADD(addr, len -
hdrm_overlap), hdrm_overlap);
        ...
    }
}

I have tried upgrading my DPDK version to 24.11 but the segmentation fault
still persists.

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