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

[Paolo Bonzini <pbonzini@redhat.com>]

On 26/02/2018 13:55, Igor Mammedov wrote:
>>> So how about just adding a new option --mem-share to decide if that's a
>>> private memory or shared memory? That seems much straightforward way
> Above options are legacy (which we can't remove for compat reasons),
> their replacement is 'memory-backend-file' backend which has all of
> the above including 'share' property.

More precisely, we have added "-object memory-backend-file" to avoid
proliferation of options related to memory.  Besides unifying the cases
of 1 and >1 NUMA node, using -object also has the advantage of
supporting memory hotplug.

You wrote "I find adding a backend for nonnuma pc RAM is roundabout way"
but basically the command line says "this VM has only one NUMA node,
backed by this memory object" which is a precise description of what the
VM memory looks like.

> So just add 'memdev' property to machine and reuse memory-backend-file
> with it instead of duplicating functionality in the legacy code.

That would however also have a different RAMBlock id, effectively
producing the same output as "-numa node,memdev=...".

I think this should be solved at the libvirt level.  Libvirt should
write in the migration XML cookie whether the VM is using -object or
-mem-path to declare its memory, and newly-started VMs should always use
-object.  This won't fix the problem for VMs that are already running,
but it will fix it the next time they are started.

Paolo