netdev - Re: bnxt_en: XDP_REDIRECT DMA unmap warnings (bnxt_tx_int

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Message-ID: <a7452b36-11cd-41b8-a82a-9ec00ab01866@hetzner-cloud.de>
Date: Wed, 6 Nov 2024 16:02:46 +0100
From: Tobias Böhm <tobias.boehm@...zner-cloud.de>
To: Michael Chan <michael.chan@...adcom.com>
Cc: netdev@...r.kernel.org
Subject: Re: bnxt_en: XDP_REDIRECT DMA unmap warnings (bnxt_tx_int_xdp)

Hi,

I managed to find a somewhat simple way to reproduce the warnings.

In my original mail I forgot to mention that the issue was observed with 
a tap device that belongs to a QEMU VM. It was reproducible with virtio 
(with and without vhost-net) and e1000 driver.

To simplify the setup I replaced QEMU with a very simple echo program 
that sends packets on a tap device. First, I just used simple 
synchronous IO, but couldn't reproduce the issue. When I switched to 
asynchronous IO by using io_uring I could reproduce the issue 
immediately. QEMU seems to use aio for the tap devices.

No other traffic on the bnxt_en interface was needed to reproduce the 
issue. Instead, the updated requirements for reproducing the issue are:
- asynchronous IO is used to write to tap device file descriptor
- packets are longer than 128 byte
- XDP_REDIRECT from tap interface to bnxt_en interface

A detailed step by step guide for reproducing the issue and the simple 
tap echo program can be found in a github repo:
https://github.com/hetznercloud/bnxt_en_xdp_redirect_reproducer/tree/main/tx-dma-unmap-issue#how-to-reproduce

The issue was reproduced with 6.11, 6.8 and 6.6 in-tree driver and also 
with latest bnxt_en out-of-tree driver provided by the vendor.

Regards, Tobias

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