virtio-mem: support VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE

The initial virtio-mem spec states that while unplugged memory should not
be read, the device still has to allow for reading unplugged memory inside
the usable region. The primary motivation for this default handling was
to simplify bringup of virtio-mem, because there were corner cases where
Linux might have accidentially read unplugged memory inside added Linux
memory blocks.

In the meantime, we:
1. Removed /dev/kmem in commit bbcd53c96071 ("drivers/char: remove
/dev/kmem for good")
2. Disallowed access to virtio-mem device memory via /dev/mem in
commit 2128f4e21aa2 ("virtio-mem: disallow mapping virtio-mem memory via
/dev/mem")
3. Sanitized access to virtio-mem device memory via /proc/kcore in
commit 0daa322b8ff9 ("fs/proc/kcore: don't read offline sections,
logically offline pages and hwpoisoned pages")
4. Sanitized access to virtio-mem device memory via /proc/vmcore in
commit ce2814622e84 ("virtio-mem: kdump mode to sanitize /proc/vmcore
access")

"Accidential" access to unplugged memory is no longer possible; we can
support the new VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE feature that will be
required by some hypervisors implementing virtio-mem in the near future.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Marek Kedzierski <mkedzier@redhat.com>
Cc: Hui Zhu <teawater@gmail.com>
Cc: Sebastien Boeuf <sebastien.boeuf@intel.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: Wei Yang <richard.weiyang@linux.alibaba.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

virtio_mem: correct tags for config space fields

Since this is a modern-only device,
tag config space fields as having little endian-ness.

TODO: check other uses of __virtioXX types in this header,
should probably be __leXX.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>

virtio_mem: convert device block size into 64bit

If subblock size is large (e.g. 1G) 32 bit math involving it
can overflow. Rather than try to catch all instances of that,
let's tweak block size to 64 bit.

It ripples through UAPI which is an ABI change, but it's not too late to
make it, and it will allow supporting >4Gbyte blocks while might
become necessary down the road.

Fixes: 5f1f79bbc9e26 ("virtio-mem: Paravirtualized memory hotplug")
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>

virtio-mem: Don't rely on implicit compiler padding for requests

The compiler will add padding after the last member, make that explicit.
The size of a request is always 24 bytes. The size of a response always
10 bytes. Add compile-time checks.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: teawater <teawaterz@linux.alibaba.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20200515101402.16597-1-david@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

virtio-mem: Allow to specify an ACPI PXM as nid

We want to allow to specify (similar as for a DIMM), to which node a
virtio-mem device (and, therefore, its memory) belongs. Add a new
virtio-mem feature flag and export pxm_to_node, so it can be used in kernel
module context.

Acked-by: Michal Hocko <mhocko@suse.com> # for the export
Acked-by: "Rafael J. Wysocki" <rafael@kernel.org> # for the export
Acked-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Tested-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Len Brown <lenb@kernel.org>
Cc: linux-acpi@vger.kernel.org
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20200507140139.17083-4-david@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

virtio-mem: Paravirtualized memory hotplug

Each virtio-mem device owns exactly one memory region. It is responsible
for adding/removing memory from that memory region on request.

When the device driver starts up, the requested amount of memory is
queried and then plugged to Linux. On request, further memory can be
plugged or unplugged. This patch only implements the plugging part.

On x86-64, memory can currently be plugged in 4MB ("subblock") granularity.
When required, a new memory block will be added (e.g., usually 128MB on
x86-64) in order to plug more subblocks. Only x86-64 was tested for now.

The online_page callback is used to keep unplugged subblocks offline
when onlining memory - similar to the Hyper-V balloon driver. Unplugged
pages are marked PG_offline, to tell dump tools (e.g., makedumpfile) to
skip them.

User space is usually responsible for onlining the added memory. The
memory hotplug notifier is used to synchronize virtio-mem activity
against memory onlining/offlining.

Each virtio-mem device can belong to a NUMA node, which allows us to
easily add/remove small chunks of memory to/from a specific NUMA node by
using multiple virtio-mem devices. Something that works even when the
guest has no idea about the NUMA topology.

One way to view virtio-mem is as a "resizable DIMM" or a DIMM with many
"sub-DIMMS".

This patch directly introduces the basic infrastructure to implement memory
unplug. Especially the memory block states and subblock bitmaps will be
heavily used there.

Notes:
- In case memory is to be onlined by user space, we limit the amount of
offline memory blocks, to not run out of memory. This is esp. an
issue if memory is added faster than it is getting onlined.
- Suspend/Hibernate is not supported due to the way virtio-mem devices
behave. Limited support might be possible in the future.
- Reloading the device driver is not supported.

Reviewed-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Tested-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Len Brown <lenb@kernel.org>
Cc: linux-acpi@vger.kernel.org
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20200507140139.17083-2-david@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>