i40e: fix potential RX buffer starvation for AF_XDP [Linux 5.0]

i40e: fix potential RX buffer starvation for AF_XDP [Linux 5.0]

This Linux kernel change "i40e: fix potential RX buffer starvation for AF_XDP" is included in the Linux 5.0 release. This change is authored by Magnus Karlsson <magnus.karlsson [at] intel.com> on Tue Jan 29 15:03:17 2019 +0100. The commit for this change in Linux stable tree is 14ffeb5 (patch).

i40e: fix potential RX buffer starvation for AF_XDP

When the RX rings are created they are also populated with buffers
so that packets can be received. Usually these are kernel buffers,
but for AF_XDP in zero-copy mode, these are user-space buffers and
in this case the application might not have sent down any buffers
to the driver at this point. And if no buffers are allocated at ring
creation time, no packets can be received and no interrupts will be
generated so the NAPI poll function that allocates buffers to the
rings will never get executed.

To rectify this, we kick the NAPI context of any queue with an
attached AF_XDP zero-copy socket in two places in the code. Once
after an XDP program has loaded and once after the umem is registered.
This take care of both cases: XDP program gets loaded first then AF_XDP
socket is created, and the reverse, AF_XDP socket is created first,
then XDP program is loaded.

Fixes: 0a714186d3c0 ("i40e: add AF_XDP zero-copy Rx support")
Signed-off-by: Magnus Karlsson <[email protected]>
Tested-by: Andrew Bowers <[email protected]>
Signed-off-by: Jeff Kirsher <[email protected]>

There are 18 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 drivers/net/ethernet/intel/i40e/i40e_main.c | 13 ++++++++++++-
 drivers/net/ethernet/intel/i40e/i40e_xsk.c  |  5 +++++
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/intel/i40e/i40e_main.c b/drivers/net/ethernet/intel/i40e/i40e_main.c
index f52e2c4..3a0990d 100644
--- a/drivers/net/ethernet/intel/i40e/i40e_main.c
+++ b/drivers/net/ethernet/intel/i40e/i40e_main.c
@@ -3289,8 +3289,11 @@ static int i40e_configure_rx_ring(struct i40e_ring *ring)
         i40e_alloc_rx_buffers_zc(ring, I40E_DESC_UNUSED(ring)) :
         !i40e_alloc_rx_buffers(ring, I40E_DESC_UNUSED(ring));
    if (!ok) {
+       /* Log this in case the user has forgotten to give the kernel
+        * any buffers, even later in the application.
+        */
        dev_info(&vsi->back->pdev->dev,
-            "Failed allocate some buffers on %sRx ring %d (pf_q %d)\n",
+            "Failed to allocate some buffers on %sRx ring %d (pf_q %d)\n",
             ring->xsk_umem ? "UMEM enabled " : "",
             ring->queue_index, pf_q);
    }
@@ -11895,6 +11898,14 @@ static int i40e_xdp_setup(struct i40e_vsi *vsi,
    if (old_prog)
        bpf_prog_put(old_prog);

+   /* Kick start the NAPI context if there is an AF_XDP socket open
+    * on that queue id. This so that receiving will start.
+    */
+   if (need_reset && prog)
+       for (i = 0; i < vsi->num_queue_pairs; i++)
+           if (vsi->xdp_rings[i]->xsk_umem)
+               (void)i40e_xsk_async_xmit(vsi->netdev, i);
+
    return 0;
 }

diff --git a/drivers/net/ethernet/intel/i40e/i40e_xsk.c b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
index 870cf654..3827f16 100644
--- a/drivers/net/ethernet/intel/i40e/i40e_xsk.c
+++ b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
@@ -183,6 +183,11 @@ static int i40e_xsk_umem_enable(struct i40e_vsi *vsi, struct xdp_umem *umem,
        err = i40e_queue_pair_enable(vsi, qid);
        if (err)
            return err;
+
+       /* Kick start the NAPI context so that receiving will start */
+       err = i40e_xsk_async_xmit(vsi->netdev, qid);
+       if (err)
+           return err;
    }

    return 0;

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