Re: [Qemu-devel] [RFC] exec: eliminate ram naming issue as migration

[Igor Mammedov <imammedo@redhat.com>]

On Mon, 5 Feb 2018 17:19:09 +0100
Paolo Bonzini <pbonzini@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
>