netdev - [PATCH net 2/2] i40e, xsk: clear the status bits for the next_to

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Date:   Fri, 11 Dec 2020 15:57:12 +0100
From:   Björn Töpel <bjorn.topel@...il.com>
To:     intel-wired-lan@...ts.osuosl.org
Cc:     Björn Töpel <bjorn.topel@...el.com>,
        magnus.karlsson@...el.com, netdev@...r.kernel.org,
        bpf@...r.kernel.org, maciej.fijalkowski@...el.com
Subject: [PATCH net 2/2] i40e, xsk: clear the status bits for the next_to_use descriptor

From: Björn Töpel <bjorn.topel@...el.com>

On the Rx side, the next_to_use index points to the next item in the
HW ring to be refilled/allocated, and next_to_clean points to the next
item to potentially be processed.

When the HW Rx ring is fully refilled, i.e. no packets has been
processed, the next_to_use will be next_to_clean - 1. When the ring is
fully processed next_to_clean will be equal to next_to_use. The latter
case is where a bug is triggered.

If the next_to_use bits are not cleared, and the "fully processed"
state is entered, a stale descriptor can be processed.

The skb-path correctly clear the status bit for the next_to_use
descriptor, but the AF_XDP zero-copy path did not do that.

This change adds the status bits clearing of the next_to_use
descriptor.

Fixes: 3b4f0b66c2b3 ("i40e, xsk: Migrate to new MEM_TYPE_XSK_BUFF_POOL")
Signed-off-by: Björn Töpel <bjorn.topel@...el.com>
---
 drivers/net/ethernet/intel/i40e/i40e_xsk.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/intel/i40e/i40e_xsk.c b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
index 567fd67e900e..e402c62eb313 100644
--- a/drivers/net/ethernet/intel/i40e/i40e_xsk.c
+++ b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
@@ -219,8 +219,11 @@ bool i40e_alloc_rx_buffers_zc(struct i40e_ring *rx_ring, u16 count)
 	} while (count);

 no_buffers:
-	if (rx_ring->next_to_use != ntu)
+	if (rx_ring->next_to_use != ntu) {
+		/* clear the status bits for the next_to_use descriptor */
+		rx_desc->wb.qword1.status_error_len = 0;
 		i40e_release_rx_desc(rx_ring, ntu);
+	}

 	return ok;
 }
-- 
2.27.0