Merge branch 'virtio-mem' into features

David Hildenbrand says:

====================
virtio-mem: s390 support

Let's finally add s390 support for virtio-mem; my last RFC was sent
4 years ago, and a lot changed in the meantime.

The latest QEMU series is available at [1], which contains some more
details and a usage example on s390 (last patch).

There is not too much in here: The biggest part is querying a new diag(500)
STORAGE_LIMIT hypercall to obtain the proper "max_physmem_end".

The last three patches are not strictly required but certainly nice-to-have.

Note that -- in contrast to standby memory -- virtio-mem memory must be
configured to be automatically onlined as soon as hotplugged. The easiest
approach is using the "memhp_default_state=" kernel parameter or by using
proper udev rules. More details can be found at [2].

I have reviving+upstreaming a systemd service to handle configuring
that on my todo list, but for some reason I keep getting distracted ...

I tested various things, including:
 * Various memory hotplug/hotunplug combinations
 * Device hotplug/hotunplug
 * /proc/iomem output
 * reboot
 * kexec
 * kdump: make sure we properly enter the "kdump mode" in the virtio-mem
   driver

kdump support for virtio-mem memory on s390 will be sent out separately.

v2 -> v3
* "s390/kdump: make is_kdump_kernel() consistently return "true" in kdump
   environments only"
 -> Sent out separately [3]
* "s390/physmem_info: query diag500(STORAGE LIMIT) to support QEMU/KVM memory
   devices"
 -> No query function for diag500 for now.
 -> Update comment above setup_ident_map_size().
 -> Optimize/rewrite diag500_storage_limit() [Heiko]
 -> Change handling in detect_physmem_online_ranges [Alexander]
 -> Improve documentation.
* "s390/sparsemem: provide memory_add_physaddr_to_nid() with CONFIG_NUMA"
 -> Added after testing on systems with CONFIG_NUMA=y

v1 -> v2:
* Document the new diag500 subfunction
* Use "s390" instead of "s390x" consistently

[1] https://lkml.kernel.org/r/20241008105455.2302628-1-david@redhat.com
[2] https://virtio-mem.gitlab.io/user-guide/user-guide-linux.html
[3] https://lkml.kernel.org/r/20241023090651.1115507-1-david@redhat.com
====================

Link: https://lore.kernel.org/r/20241025141453.1210600-1-david@redhat.com/
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>