Re: [Qemu-devel] Live migration + cpu/mem hotplug

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

On Thu, Jan 05, 2017 at 04:27:26PM +0800, Bob Chen wrote:
> Hi,
> 
> According to the docs, the destination Qemu must have the exactly same
> parameters as the source one. So if the source has just finished cpu or
> memory hotplug, what would the dest's parameters be like?
> 
> Does DIMM device, or logically QOM object, have to be reflected on the new
> command-line parameters?

Yes, if you have hotplugged any type of device since the VM was started,
the QEMU command line args on the target host must include all the original
args from the source QEMU, and also any args reflect to reflect the
hotplugged devices too.

A further complication is that on the target, you must also make sure you
fully specify *all* device address information (PCI slots, SCSI luns, etc
etc), because the addresses QEMU assigns to a device after hotplug may
not be the same as the addresses QEMU assigns to a device whne coldplug.

eg if you boot a guest with 1 NIC + 1 disk, and then hotplug a 2nd NIC
you might get

   1st NIC  == PCI slot 2
   1st disk == PCI slot 3
   2nd NIC  == PCI slot 4

if however, you started QEMU with 2 NICs and 1 disk straight away QEMU
might assign addresses in the order

   1st NIC  == PCI slot 2
   2nd NIC  == PCI slot 3
   1st disk == PCI slot 4

this would totally kill a guest OS during live migration as the slots
for devices its using would change.

So as a general rule when launching QEMU on a target host for migrating,
you must be explicit about all device addresses and not rely on QEMU to
auto-assign addresses. This is quite alot of work to get right, but if
you're using libvirt it'll do pretty much all this automatically for
you.

Regards,
Daniel
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