From an architecture point of view, nothing can be mapped into the address
space on s390x. All there is is memory. Therefore there is also not really
an interface to communicate such information to the guest. All we can do is
specify the maximum ram address and guests can probe in that range if
memory is available and usable (TPROT).
Also memory hotplug is strange. The guest can decide at some point in
time to add / remove memory in some range. While the hypervisor can deny
to online an increment, all increments have to be predefined and there is
no way of telling the guest about a newly "hotplugged" increment. So if we
specify right now e.g.
-m 2G,slots=2,maxmem=20G
An ordinary fedora guest will happily online (hotplug) all memory,
resulting in a guest consuming 20G. So it really behaves rather like
-m 22G
There is no way to hotplug memory from the outside like on other
architectures. This is of course bad for upper management layers.
As the guest can create/delete memory regions while it is running, of
course migration support is not available and tricky to implement.
With virtualization, it is different. We might want to map something
into guest address space (e.g. fake DAX devices) and not detect it
automatically as memory. So we really want to use the maxmem and slots
parameter just like on all other architectures. Such devices will have
to expose the applicable memory range themselves. To finally be able to
provide memory hotplug to guests, we will need a new paravirtualized
interface to do that (e.g. something into the direction of virtio-mem).
This implies, that maxmem cannot be used for s390x memory hotplug
anymore and has to go. This simplifies the code quite a bit.
As migration support is not working, this change cannot really break
migration as guests without slots and maxmem don't see the SCLP
features. Also, the ram size calculation does not change.
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180219174231.10874-1-david@redhat.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Matthew Rosato <mjrosato@linux.vnet.ibm.com>
[CH: tweaked patch description, as discussed on list]
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
82fab5c5b9