On Thu, Jul 16, 2026 at 01:11:05AM +0300, Vladimir Sementsov-Ogievskiy wrote:
> On 15.07.26 23:32, Michael S. Tsirkin wrote:
> > On Wed, Jul 15, 2026 at 11:21:23PM +0300, Vladimir Sementsov-Ogievskiy
> > wrote:
> > > On 15.07.26 19:00, Peter Xu wrote:
> > > > On Wed, Jul 15, 2026 at 04:52:48PM +0100, Daniel P. Berrangé wrote:
> > > > > On Wed, Jul 15, 2026 at 11:48:40AM -0400, Michael S. Tsirkin wrote:
> > > > > > On Wed, Jul 15, 2026 at 06:01:39PM +0300, Michael Tokarev wrote:
> > > > > > > On 14.07.2026 18:42, Vladimir Sementsov-Ogievskiy wrote:
> > > > > > > > Hi all!
> > > > > > > >
> > > > > > > > Here is a migration for TAP net backend, including its
> > > > > > > > properties and
> > > > > > > > open fds.
> > > > > > > >
> > > > > > > > With this new feature, management software doesn't need to
> > > > > > > > initialize
> > > > > > > > new TAP and do a switch to it. Nothing should be done around
> > > > > > > > virtio-net in local migration: it just migrates and continues
> > > > > > > > to use
> > > > > > > > same TAP device. So we avoid extra logic in management
> > > > > > > > software, extra
> > > > > > > > allocations in kernel (for new TAP), and corresponding extra
> > > > > > > > delay in
> > > > > > > > migration downtime.
> > > > > > >
> > > > > > > This is quite a big patch set, - is this really worth the effort
> > > > > > > to do
> > > > > > > all this just for *local* migration? What's the possible use
> > > > > > > case for
> > > > > > > this in real, - am I right this is just about upgrading the host
> > > > > > > qemu?
> > > > > > > And with that in mind, isn't it sufficient to use what we already
> > > > > > > have
> > > > > > > (namely, create new tap, start new qemu instance, and migrate the
> > > > > > > usual
> > > > > > > way), and tolerate some very minor downtime while the networking
> > > > > > > code
> > > > > > > learns the new network topology (isn't it happening almost
> > > > > > > instantly
> > > > > > > anyway, and if not, the management can help by sending gratitious
> > > > > > > ARP)?
> > > > > > >
> > > > > > > I wonder what's the use for this at yandex?
> > > > > > >
> > > > > > > Thanks,
> > > > > > >
> > > > > > > /mjt
> > > > > >
> > > > > > Well just theoretically, imagine a big VM, reserving twice the
> > > > > > amount
> > > > > > of memory just to migrate is not nice at all.
> > > > >
> > > > > Don't we already have the ability to skip memory transfer by setting
> > > > > the "x-ignore-shared" capability, assuming the VM RAM has a shared
> > > > > memory backing.
> > > >
> > > > Right, IIUC all similar single-host migrations like this series or CPR
> > > > (or
> > > > anything else...) should always need to enable x-ignore-shared in the
> > > > first
> > > > place. That's almost always the starting point of optimizing local
> > > > migrations.. no matter how the memory will be shared (by the same pool
> > > > of
> > > > page cache, or persisted over kexec, etc.).
> > > >
> > >
> > > Yes, sharing RAM between source and target + enabling x-ignore-shared is
> > > a first thing to do.
> > >
> > > This series optimizes TAP recreating. Not only skip recreating but also
> > > allow to
> > > exclude cloud-networking component form live-update entirely, making the
> > > process
> > > simpler (less components involved), and as I already said, reducing
> > > corresponding
> > > downtime.
> > >
> > > --
> > > Best regards,
> > > Vladimir
> >
> > So can you explain, how is this better than
> > 1. a persistent tap
>
> You mean open same tap device both on source and target?
>
> This will require some additional steps anyway, to avoid packet loss,
> like keeping queues disabled on target until post_load.
>
> Another thing is MAX_TAP_QUEUES=256 in kernel: this is a problem, if you have
> more
> than 128 queues already opened on source. Seems cleaner just pass already
> opened
> queues to the target.
>
> And finally, on hosts with many CPUs, TAP queue allocate noticeable amount
> of RAM, so having x2 queues during migration would be an overhead.
ah I forgot. yes queues do get
> > 2. a non persistent tap that some server gets a hold of
>
> You mean, just pass FDs externally, instead of using QEMUs migration stream?
> That's possible. But requires mgmt tool to store (or get from source) and pass
> these FDs. Requires mgmt to even know about these FDs.
e.g. libvirt already does, right? it creates them?
> But why? QEMU already
> can pass FDs through migration for vfio devices (CPR), why is TAP worse?
it's not that it's worse. it's that we are growing bespoke mechanisms so far.
so if qemu gets tap fd on command line then what? how does that
interact?
> Like with persistent tap, it will require some changes in Qemu anyway, to
> avoid packet loss
> (like patch 12/15 here).
that one is more like a bugfix.
> >
> > IOW why does qemu need to bother.
>
> QEMU owns the TAP fd and has full knowledge of its state. Pushing this
> responsibility
> to an external tool means the tool needs to understand QEMU internals just to
> pass
> an fd that QEMU already has. I think, that's a worse separation of concerns.
>
> Of course, there are other ways to do TAP local migration. But looking at
> wider
> picture, where we want to migrate not only TAP, but also vfio devices (already
> implemented as CPR migration, but may be updated to use similar approach as in
> this series, to use one migration channel), vhost-user-blk (my another series
> in flight) and vhost-user-fs (not yet published), vhost-vsock migration
> ("[PATCH v3 0/7] migration/cpr: support vhost-vsock devices" in flight
> series),
> it seems a good generic approach: simply pass backends (including open FDs) to
> the target, not involving mgmt. Qemu has full knowledge about these FDs and
> owns the whole state. Migrating them in QEMU looks correct for me.
if you find a way to generalize things and reuse them for your
purposes without intrusive changes all over qemu, fine.
But this one is poking as far as frontend code even.
> --
> Best regards,
> Vladimir
