On Mon, Feb 27, 2017 at 11:02:09AM +0100, Vitaly Kuznetsov wrote:

A couple of other thoughts:
1) Having all newly added memory online ASAP is probably what people
want for all virtual machines.

This is not true for s390. On s390 we have "standby" memory that a guest
sees and potentially may use if it sets it online. Every guest that sets
memory offline contributes to the hypervisor's standby memory pool, while
onlining standby memory takes memory away from the standby pool.

The use-case is that a system administrator in advance knows the maximum
size a guest will ever have and also defines how much memory should be used
at boot time. The difference is standby memory.

Auto-onlining of standby memory is the last thing we want.

Unfortunately, we have additional complexity with memory zones
(ZONE_NORMAL, ZONE_MOVABLE) and in some cases manual intervention is
required. Especially, when further unplug is expected.

This also is a reason why auto-onlining doesn't seem be the best way.