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

[Paolo Bonzini]

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.

>> 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