id = 2745
title = "Qemu should send RARP for vhostuser regardless of whether virtio supports GUEST_ANNOUNCE"
state = "opened"
created_at = "2024-12-23T16:32:49.159Z"
closed_at = "n/a"
labels = ["Networking", "device:virtio"]
url = "https://gitlab.com/qemu-project/qemu/-/issues/2745"
host-os = "RHEL"
host-arch = "any"
qemu-version = "- QEMU command line:"
guest-os = "- OS/kernel version:"
guest-arch = "## Description of problem"
description = """When virtio reports to qemu that GUEST_ANNOUNCE is supported, qemu will not send RARP. The assumption, I think, is that guest kernel will do whatever is needed to announce the guest. The problem with this assumption is that 1) kernel won't send RARPs; 2) for IPv4 and IPv6 it will send GARPs and NAs; 3) for interfaces with no IP addresses, I think it won't send anything at all.

RARPs are useful because they allow to notify regardless of IP configuration. I've [asked](https://issues.redhat.com/browse/RHEL-71919]) RHEL kernel folks to consider issuing RARPs from the guest kernel, but it won't happen overnight, and regardless, it's not a complete solution since we cannot expect all guests running a patched kernel with such feature.

RARP packets are also often expected by underlying network components. For example, OVN controller has a special "activation-strategy=rarp" configuration that makes OVN wait for a RARP from destination chassis on live migration, and only then unblock traffic for the port. Since RARP is not issed by Qemu nor virtio-net, the OVN port is never unblocked (until its configuration is reset by CMS).

I think what should be done from Qemu side is to send RARP for vhostuser ports regardless of whether virtio supports GUEST_ANNOUNCE. I **think** this can be achieved by removing this code:

```
    /* If guest supports GUEST_ANNOUNCE do nothing */
    if (virtio_has_feature(dev->acked_features, VIRTIO_NET_F_GUEST_ANNOUNCE)) {
        return 0;
    }
```"""
reproduce = """1. Start a VM with vhostuser* port and fresh virtio guest driver.
2. Live migrate it.
3. Observe that RARP is not sent from the migrated port. GARP (or NA for IPv6) is sent instead."""
additional = """Some external bugs that may be relevant:

- RHEL kernel request to send RARPs from virtio: https://issues.redhat.com/browse/RHEL-71919 (won't fix the issue for older unpatched kernels)
- Request for OVN to handle GARPs and NAs: https://issues.redhat.com/browse/FDP-1042 (won't solve for unaddressed ports!)
- An attempt to work around the issue in OpenStack Neutron OVN env by disabling activation strategy: https://issues.redhat.com/browse/OSPRH-12571 (not a great long term solution)"""