Re: [PATCH] migration: Apply migration specific keep-alive defaults to inet socket

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Sep 18, 2025 at 11:10:49AM -0400, Peter Xu wrote:
> On Thu, Sep 18, 2025 at 03:45:21PM +0100, Daniel P. Berrangé wrote:
> > There needs to be a way to initiate post-copy recovery regardless
> > of whether we've hit a keepalive timeout. Especially if we can
> > see one QEMU in postcopy-paused, but not the other side, it
> > doesn't appear to make sense to block the recovery process.
> > 
> > The virDomainJobCancel command can do a migrate-cancel on the
> > src, but it didn't look like we could do the same on the dst.
> > Unless I've overlooked something, Libvirt needs to gain a way
> > to explicitly force both sides into the postcopy-paused state,
> > and thus be able to immediately initiate recovery.
> 
> Right, if libvirt can do that then problem should have been solved too.
> 
> > I'm fine with turning on keepalives on the socket, but IMHO the
> > out of the box behaviour should be to honour the kernel default
> > tunables unless the admin decides they want different behaviour.
> > I'm not seeing a rational for why the kernel defaults should be
> > forceably overridden in QEMU out of the box.
> 
> IMHO the rational here is that the socket here is in a special state and
> for special purpose. So we're not trying to change anything globally for
> qemu (without knowing what the socket is), but only this specific type of
> socket that is used for either precopy or postcopy live migrations.
> 
> It's special because it's always safe to have a more aggresive
> disconnection, and might be preferred versus very lengthy hangs (if
> assuming libvirt doesn't yet have way to stop the hang), especially for a
> postcopy phase.

I've already described up-thread that it is not guaranteed safe to
have aggressive disconnection. It is often safe, but there is
definitely non-negligible risk in prematurely terminating a migration
connection.

With regards,
Daniel
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