vtnet: disable hardware TCP LRO by default
ClosedPublic
Actions

Authored by tuexen on Mon, Sep 22, 8:37 PM.

Details

Reviewers

timo.voelker_fh-muenster.de
bryanv
kp

Group Reviewers

manpages

Commits

rGe1a7840dd941: vtnet: disable hardware TCP LRO by default
rG3d548504c705: vtnet: disable hardware TCP LRO by default
rG6e4b811009d6: vtnet: disable hardware TCP LRO by default

Summary

Hardware TCP LRO results in problems in settings with IP forwarding being enabled. In case of nodes without IP forwarding, using software LRO is also beneficial in general, since it can provide better information about what was received on the wire.
Therefore, disable hardware TCP LRO by defaults. By tuning the loader tunable, this can be changed.

Diff Detail

Repository

rG FreeBSD src repository

Lint

Lint Not Applicable

Unit

Tests Not Applicable

Event Timeline

tuexen created this revision.Mon, Sep 22, 8:37 PM

Herald added subscribers: ziaee, imp. · View Herald TranscriptMon, Sep 22, 8:37 PM

tuexen requested review of this revision.Mon, Sep 22, 8:37 PM

I did some tests to understand the consequence of disabling "hardware LRO".

If the guest disables hardware LRO, the host cannot send an oversized packet to the guest. If the host receives an oversized packet - from another guest whose driver used TSO or from a physical interface whose driver performed LRO - it must perform TSO in software before it can send the resulting smaller segments to the guest. This makes the packet transfer less efficient.

It's easier for the host if the guest has hardware LRO enabled. However, the big issue is that a FreeBSD guest cannot route and send an oversized packet out via another interface. It could use TSO of the outgoing interface or perform TSO in software if the outgoing interface does not offer TSO, but this is not implemented, and implementing it needs some time. Until then, disabling hardware LRO seems to be the better default setting.