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

[Bob Chen]

Answer my own question:

The corresponding cmd-line parameter for memory hot-add by QEMU monitor is,
-object memory-backend-ram,id=mem0,size=1024M -device
pc-dimm,id=dimm0,memdev=mem0

2017-01-05 18:12 GMT+08:00 Daniel P. Berrange <address@hidden>:

> 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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