Re: [Qemu-devel] [PATCH v8 4/8] ACPI: Add Virtual Machine Generation ID support

[Dr. David Alan Gilbert]

* Eric Blake (address@hidden) wrote:
> On 02/20/2017 04:23 AM, Dr. David Alan Gilbert wrote:
> > * Laszlo Ersek (address@hidden) wrote:
> >> CC Dave
> > 
> > This isn't an area I really understand; but if I'm
> > reading this right then 
> >    vmgenid is stored in fw_cfg?
> >    fw_cfg isn't migrated
> > 
> > So why should any changes to it get migrated, except if it's already
> > been read by the guest (and if the guest reads it again aftwards what's
> > it expected to read?)
> 
> Why are we expecting it to change on migration?  You want a new value

I'm not; I was asking why a change made prior to migration would be
preserved across migration.


> when you load state from disk (you don't know how many times the same
> state has been loaded previously, so each load is effectively forking
> the VM and you want a different value), but for a single live migration,
> you aren't forking the VM and don't need a new generation ID.
> 
> I guess it all boils down to what command line you're using: if libvirt
> is driving a live migration, it will request the same UUID in the
> command line of the destination as what is on the source; while if
> libvirt is loading from a [managed]save to restore state from a file, it
> will either request a new UUID directly or request auto to let qemu
> generate the new id.

Hmm now I've lost it a bit; I thought we would preserve the value
transmitted from the source, not the value on the command line of the 
destination.

Dave

> 
> -- 
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org
> 



--
Dr. David Alan Gilbert / address@hidden / Manchester, UK