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

[Tan, Jianfeng]

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.

> >       RAMBLOCK_FOREACH(block) {
> > -        if (!strcmp(name, block->idstr)) {
> > +        name2 = strdup(block->idstr);
> > +       id2 = basename(name2);
> > +        if (!strcmp(id1, id2)) {
> > +            free(name1);
> > +           free(name2);
> >               return block;
> >           }
> > +       free(name2);
> Instead of this, perhaps just skip "/object/" in both id1 and
> block->idstr?  This also removes the need for strdup/free.

Not familiar with this before, so there would not be something like 
/object/xxx/id? In that case, we can just skip "/object/". Thanks!

> 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.

Thanks,
Jianfeng

> Thanks,
>
> Paolo
>
> >       }
> >   
> > +    free(name1);
> >       return NULL;
> >   }
> >