Re: [PATCH 0/4] migration: Pass network packets received during switchover to dest VM

[Stefano Brivio <sbrivio@redhat.com>]

[Cc'ing Laurent and David]

On Tue, 27 Jan 2026 15:03:06 +0100
Juraj Marcin <jmarcin@redhat.com> wrote:

> During switchover there is a period during which both source and
> destination side VMs are paused. During this period, all network packets
> are still routed to the source side, but it will never process them.
> Once the destination resumes, it is not aware of these packets and they
> are lost. This can cause packet loss in unreliable protocols and
> extended delays due to retransmission in reliable protocols.
> 
> This series resolves this problem by caching packets received once the
> source VM pauses and then passing and injecting them on the destination
> side. This feature is implemented in the last patch. The caching and
> injecting is implemented using network filter interface and should work
> with any backend with vhost=off, but only TAP network backend was
> explicitly tested.

I haven't had a chance to try this change with passt(1) yet (the
backend can be enabled using "-net passt" or by starting it
separately).

Given that passt implements migration on its own (in deeper detail in
some sense, as TCP connections are preserved if IP addresses match), I
wonder if it this might affect or break it somehow.

Did you perhaps have some thoughts about that already?

For context, we didn't really write comprehensive documentation about
it yet, but:

- KubeVirt's enhancement repository has a detailed description at:
  https://github.com/kubevirt/enhancements/blob/main/veps/sig-network/passt/passt-migration-proposal.md#live-migration-with-passt

- the QEMU-facing details are outlined in:
  https://archives.passt.top/passt-dev/20241219111400.2352110-1-lvivier@redhat.com/

- usage of TCP_REPAIR is briefly described in passt-repair(1)

-- 
Stefano