id = 1312
title = "TCP performance problems - GSO/TSO, MSS, 8139 related (Ignores lower MTU from PMTUD/MSS)"
state = "closed"
created_at = "2022-11-13T22:52:41.955Z"
closed_at = "2022-11-21T21:21:31.810Z"
labels = ["Networking"]
url = "https://gitlab.com/qemu-project/qemu/-/issues/1312"
host-os = "-"
host-arch = "any"
qemu-version = "- QEMU command line:"
guest-os = "Linux (others not tested)"
guest-arch = "Any"
description = """MTU handling on guests using an RTL8139 virtualized NIC is broken; net/hw/8139.c works with a static MTU of 1500b for TCP offloading, leading to low throughput when clients connect from sub 1500MTU networks. PMTUD is ignored, and locking to a lower MTU in the OS mitigates the issue."""
reproduce = """1. Create a guest with an RTL8139 nic
2. Try to retrieve a file from a client behind a sub 1500 MTU link
3. Observe low bandwidth due to retransmits"""
additional = """I just debugged this issue for an NGO which, for whatever reason, had an RTL8139 NIC in their guest. After i finally traced this to the RTL8139, i found this qemu-devel/netdev thread from six years ago, which apparently already debugged this issue and proposed a patch: https://lore.kernel.org/all/20161114162505.GD26664@stefanha-x1.localdomain/

I did not test the patch proposed there, but note that `net/hw/8139.c` still looks as discussed in that qemu-devel/netdev thread. As i haven't found a bug report in the archives, i figured you might want to know."""