On Mon, 5 Feb 2018 17:19:09 +0100 Paolo Bonzini <pbonz...@redhat.com> wrote:
> On 05/02/2018 17:12, Tan, Jianfeng wrote: > > Hi Paolo, > > > > On 2/5/2018 11:53 PM, Paolo Bonzini wrote: > >> On 05/02/2018 15:58, Jianfeng Tan wrote: > >>> Here are some options to fix this: > >>> > >>> 1. When we do ram name comparison, we truncate the prefix as this > >>> patch shows. > >>> It cannot cover the corner case: the source VM could have two ram blocks > >>> with name of "pc.ram" and "/object/pc.ram". > >> That shouldn't happen ("pc.ram" exists even in the "-numa > >> node,memdev=..." case, but it has no RAM block). > > > > Suppose we have a VM started with "-m xG", and then hot plugged with a > > ram block: > > (qemu) object_add > > memory-backend-file,id=pc.ram,size=1G,mem-path=/dev/hugepages > > (qemu) device_add pc-dimm,id=pc.ram,memdev=pc.ram > > > > Then we would have both ram block named pc.ram: > > Block Name PSize > > pc.ram 4 KiB > > /objects/pc.ram 2 MiB > > > > But I assume it's a corner case which not really happen. > > Yeah, you're right. :/ I hadn't thought of hotplug. It can happen indeed. perhaps we should fail object_add memory-backend-foo if it resulted in creating ramblock with duplicate id > > >> However, note that > >> > >> -m xG -numa node,memdev=pc.ram \ > >> -object memory-backend-file,id=pc.ram,... > >> > >> works for both vhost-kernel and vhost-user, so I'd rather consider this > >> a configuration problem and not do anything. > > > > That configuration indeed works for both. But in the production env, > > lots of VMs are already started with previous mem config. If we do > > nothing, it will take a long time (shutdown/start for each VM) to > > migrate to the new setup. This patch is to make this process more smooth > > without any bad effect if possible. > > I understand. However it's not as bad as "there's no possibility at all > to migrate from vhost-kernel to vhost-user". There are cases that are > more problematic: for example, there's no possibility at all to add > memory NUMA policy during a live migration, unless -object > memory-backend-* was used on the source. > > Paolo >