On Mon, May 08, 2023 at 09:25:48AM -0700, Stephen Hemminger wrote: > On Mon, 8 May 2023 06:30:07 -0400 > "Michael S. Tsirkin" <m...@redhat.com> wrote: > > > > > > I don't know, in any scenario, when the hardware supports a large > > > > > mtu, but we do > > > > > not want the user to use it by default. > > > > > > > > When other devices on the same LAN have mtu set to 1500 and > > > > won't accept bigger packets. > > > > > > So, that depends on pmtu/tcp-probe-mtu. > > > > > > If the os without pmtu/tcp-probe-mtu has a bigger mtu, then it's big > > > packet > > > will lost. > > > > > > Thanks. > > > > > > > pmtu is designed for routing. LAN is supposed to be configured with > > a consistent MTU. > > Virtio is often used with bridging or macvlan which can't support PMTU. > PMTU only works when forwarding at layer 3 (ie routing) where there is > a IP address to send the ICMP response. If doing L2 forwarding, the > only thin the bridge can do is drop the packet. > > TCP cab recover but detecting an MTU blackhole requires retransmissions.
Exactly. That's why we basically use the MTU advice supplied by device by default - it's designed for use-cases of software devices where the device might have more information about the MTU than the guest. If hardware devices want e.g. a way to communicate support for jumbo frames without communicating any information about the LAN, a new feature will be needed. -- MST _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
