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

[Stefano Brivio <sbrivio@redhat.com>]

On Tue, 3 Feb 2026 13:03:26 +0100
Laurent Vivier <lvivier@redhat.com> wrote:

> On 1/27/26 19:21, Stefano Brivio wrote:
> > [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.
> 
> passt implements migration only with the vhost-user backend ("-netdev vhost-user") that is 
> not supported by netpass. All the vhost-* cannot be supported because netpass cannot catch 
> packets on the virtio queues.

Thanks for having a look! On this point... right, hence my question in:

  https://lore.kernel.org/qemu-devel/20260131032700.12f27487@elisabeth/

that is, is there a plan to add vhost support *for netpass*, eventually?
It looks like yes:

  https://lore.kernel.org/qemu-devel/CACLfguUZpT-3sj4C8G8e+LB5GHpBfE_HKLOhyZ9qYR8bgkTOCw@mail.gmail.com/

but I'm not sure I got it right (Cindy? Jason?).

> passt with "-netdev stream" doesn't implement migration, but QEMU can be migrated with it 
> and all the connections are lost. So netpass will forward packets for connections that 
> will be broken.

Realistically, I don't think anybody will ever try to migrate VMs using
-netdev stream with passt, so I guess we don't really have to care
about this (it might help with some protocols, probably make UDP usage
a bit worse, waste a bit of bandwidth with TCP... but that's it).

The only existing (known) user of passt's migration feature is
KubeVirt, which switched to passt's vhost-user interface entirely.

> "-netdev passt" is only some kind of wrapper on top of "-netdev stream" and "-netdev 
> vhost-user" that starts the passt backend by itself (rather than expecting it has been 
> started by the user).

-- 
Stefano