Re: [Qemu-devel] Runtime-modified DIMMs and live migration issue

[Andrey Korolyov]

On Thu, Jun 11, 2015 at 8:14 PM, Andrey Korolyov <address@hidden> wrote:
> Hello Igor,
>
> the current hotplug code for dimms effectively prohibiting a
> successful migration for VM if memory was added after startup:
>
> - start a VM with certain amount of empty memory slots,
> - add some dimms and online them in guest (I am transitioning from 2
> to 16G with 512Mb DIMMs),
> - migrate a VM and observe guest null pointer dereference (or BSOD
> with reboot, for Windows).
>
> Issue is currently touching all stable versions and assumingly master,
> as there are no related fixes/RFCs since 2.3 I`m currently using for
> testing. The issue is related to an incorrect population of the
> regions during runtime hotplugging, hopefully 2.4 will get the fix.
>
> You may run some workload in guest to achieve one hundred percent
> certainty of hitting the issue, for example, fio against
> http://xdel.ru/downloads/fio.txt . QEMU args are simular to '... -m
> 512,slots=31,maxmem=16384M -object
> memory-backend-ram,id=mem0,size=512M -device
> pc-dimm,id=dimm0,node=0,memdev=mem0 -object
> memory-backend-ram,id=mem1,size=512M -device
> pc-dimm,id=dimm1,node=0,memdev=mem1 -object
> memory-backend-ram,id=mem2,size=512M -device
> pc-dimm,id=dimm2,node=0,memdev=mem2...'
>
> Thanks for looking into this!

Please find the full cli args and two guest logs for DIMM
initalization attached. As you can see, the freshly populated DIMMs
are probably misplaced in SRAT ('already populated' messages), despite
the fact that the initialized ranges are looking correct at a glance.
When VM is migrated to the destination (with equal RAM device
configuration) which is simular to a VM with 16G RAM this misplacement
is causing the mentioned panic in the guest. This should be very
easily reproducible and the issue is very critical as well, I don`t
even understand why I missed this issue earlier.

hotplug-qemu-args.txt
Description: Text document

16G-no-hotplug.txt
Description: Text document

4G-hotplug-12G.txt
Description: Text document