Re: [Qemu-devel] [PATCH 1/1] migration: announce VM's new home just before VM is runnable

[Juan Quintela]

Amit Shah <address@hidden> wrote:
> On (Wed) 14 Oct 2015 [17:12:44], Michael S. Tsirkin wrote:
>> On Wed, Oct 14, 2015 at 03:21:15PM +0200, Juan Quintela wrote:
>> > Amit Shah <address@hidden> wrote:
>> > > We were announcing the dest host's IP as our new IP a bit too soon -- if
>> > > there were errors detected after this announcement was done, the
>> > > migration is failed and the VM could continue running on the src host --
>> > > causing problems later.
>> > >
>> > > Move around the qemu_announce_self() call so it's done just before the
>> > > VM is runnable.
>> > >
>> > > Signed-off-by: Amit Shah <address@hidden>
>> > 
>> > Reviewed-by: Juan Quintela <address@hidden>
>> > 
>> > applied.
>> > 
>> > I have the same question than Dave, but also agree that this is a
>> > movement in the right direction.
>> > 
>> > Why it is not only needed when we do a vm_start()?
>> 
>> It's a complex question.  We don't want to do this on each
>> vmstop/vmcont. But maybe we want to, on the 1st vmstart after
>> qemu is started.
>
> Why?  When a guest starts, it will issue ARP requests and everything
> will just work.  We need this announce_self only to tell the switches
> that the MAC belonging to the guest's IP has changed..

What happens if we stop a guest on one host and we start it on a
different host?

If the communication is started from a different place, packets will go
to old host, until some TCP timeout happens, right?

Later, Juan.