netdev - Re: [Bug][USB4NET] Potential Bug with USB4NET and Linux Bridging

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Date: Thu, 14 Sep 2023 18:51:26 +0300
From: Mika Westerberg <mika.westerberg@...ux.intel.com>
To: Yachen Liu <blankwonder@...il.com>
Cc: netdev@...r.kernel.org, michael.jamet@...el.com, YehezkelShB@...il.com
Subject: Re: [Bug][USB4NET] Potential Bug with USB4NET and Linux Bridging

Hi,

On Thu, Sep 14, 2023 at 10:02:24PM +0800, Yachen Liu wrote:
> Hello,
> 
> I've noticed a potential issue with USB4NET when used in conjunction
> with Linux bridging. I'm uncertain if it's a bug or a configuration
> oversight on my part.
> 
> In essence, when the device at the other end of Thunderbolt sends
> packets using the TSO mechanism (default behavior in macOS), the Linux
> thunderbolt0 interface correctly receives this large packet. However,
> the packet isn't properly forwarded to another device via bridging.
> 
> Detailed Description:
> 
> The test environment consists of three systems:
> 
> A: Mac Mini (M2): macOS Sonoma 14.0 RC
> B: Proxmox VE 8.0. Kernel release: 6.2.16-3-pve, acting as the Host system.
> C: Debian. A Guest system running within B.
> 
> System A and B are connected via USB4, while System C is a virtual
> machine within B. On B, thunderbolt0 and tap102i0 are bridged to
> establish a connection between A and C.
> 
> During an iperf3 speed test between A and B, I've achieved
> bi-directional speeds of around 18Gbps. Between B and C, the speeds
> are 100Gbps+ at their peak, with a minimum of 28Gbps.
> 
> However, when performing an iperf3 speed test between A and C, the
> direction from C to A shows about 18Gbps, but from A to C, the speed
> drops to just tens of Kbps, essentially making it unusable.
> 
> If tested using UDP, both directions achieve roughly 5Gbps. (I suspect
> some buffer issue in macOS limiting the speed).
> 
> After various tests and investigations, I found that by setting
> macOS's net.inet.tcp.tso to 0 (disabling TSO), speeds from A to C
> improved to around 10Gbps.
> 
> Packet capture via tcpdump revealed that macOS writes large packets
> (over 10000B) to Thunderbolt Networking using TSO. These packets are
> correctly captured on thunderbolt0, but are missing from tap102i0,
> resulting in significant packet loss.
> 
> Since ethtool doesn't support the thunderbolt0 device, further testing
> has been hindered.
> 
> I'm unsure if this is a bug, or if it could be resolved via
> configuration. If more information is needed, I am more than willing
> to assist further with tests.

Thank you for the report. To be honest, I'm not sure how the interface
should in this case and is there perhaps some way for a NIC driver to
pass TSO packets up so that they would be passed in the same way to the
tap interface. Is there any example driver where this works? Like if you
use ethernet instead of TBT/USB4 does that work with TSO packets?