[PATCH RFC 00/15] virtio-mem: Expose device memory via separate memslots

[David Hildenbrand]

Based-on: 20211011175346.15499-1-david@redhat.com

A virtio-mem device is represented by a single large RAM memory region
backed by a single large mmap.

Right now, we map that complete memory region into guest physical addres
space, resulting in a very large memory mapping, KVM memory slot, ...
although only a small amount of memory might actually be exposed to the VM.

For example, when starting a VM with a 1 TiB virtio-mem device that only
exposes little device memory (e.g., 1 GiB) towards the VM initialliy,
in order to hotplug more memory later, we waste a lot of memory on metadata
for KVM memory slots (> 2 GiB!) and accompanied bitmaps. Although some
optimizations in KVM are being worked on to reduce this metadata overhead
on x86-64 in some cases, it remains a problem with nested VMs and there are
other reasons why we would want to reduce the total memory slot to a
reasonable minimum.

We want to:
a) Reduce the metadata overhead, including bitmap sizes inside KVM but also
   inside QEMU KVM code where possible.
b) Not always expose all device-memory to the VM, to reduce the attack
   surface of malicious VMs without using userfaultfd.

So instead, expose the RAM memory region not by a single large mapping
(consuming one memslot) but instead by multiple mappings, each consuming
one memslot. To do that, we divide the RAM memory region via aliases into
separate parts and only map the aliases into a device container we actually
need. We have to make sure that QEMU won't silently merge the memory
sections corresponding to the aliases (and thereby also memslots),
otherwise we lose atomic updates with KVM and vhost-user, which we deeply
care about when adding/removing memory. Further, to get memslot accounting
right, such merging is better avoided.

Within the memslots, virtio-mem can (un)plug memory in smaller granularity
dynamically. So memslots are a pure optimization to tackle a) and b) above.

Memslots are right now mapped once they fall into the usable device region
(which grows/shrinks on demand right now either when requesting to
 hotplug more memory or during/after reboots). In the future, with
VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE, we'll be able to (un)map aliases even
more dynamically when (un)plugging device blocks.


Adding a 500GiB virtio-mem device and not hotplugging any memory results in:
    0000000140000000-000001047fffffff (prio 0, i/o): device-memory
      0000000140000000-0000007e3fffffff (prio 0, i/o): virtio-mem-memslots

Requesting the VM to consume 2 GiB results in (note: the usable region size
is bigger than 2 GiB, so 3 * 1 GiB memslots are required):
    0000000140000000-000001047fffffff (prio 0, i/o): device-memory
      0000000140000000-0000007e3fffffff (prio 0, i/o): virtio-mem-memslots
        0000000140000000-000000017fffffff (prio 0, ram): alias 
virtio-mem-memslot-0 @mem0 0000000000000000-000000003fffffff
        0000000180000000-00000001bfffffff (prio 0, ram): alias 
virtio-mem-memslot-1 @mem0 0000000040000000-000000007fffffff
        00000001c0000000-00000001ffffffff (prio 0, ram): alias 
virtio-mem-memslot-2 @mem0 0000000080000000-00000000bfffffff

Requesting the VM to consume 20 GiB results in:
    0000000140000000-000001047fffffff (prio 0, i/o): device-memory
      0000000140000000-0000007e3fffffff (prio 0, i/o): virtio-mem-memslots
        0000000140000000-000000017fffffff (prio 0, ram): alias 
virtio-mem-memslot-0 @mem0 0000000000000000-000000003fffffff
        0000000180000000-00000001bfffffff (prio 0, ram): alias 
virtio-mem-memslot-1 @mem0 0000000040000000-000000007fffffff
        00000001c0000000-00000001ffffffff (prio 0, ram): alias 
virtio-mem-memslot-2 @mem0 0000000080000000-00000000bfffffff
        0000000200000000-000000023fffffff (prio 0, ram): alias 
virtio-mem-memslot-3 @mem0 00000000c0000000-00000000ffffffff
        0000000240000000-000000027fffffff (prio 0, ram): alias 
virtio-mem-memslot-4 @mem0 0000000100000000-000000013fffffff
        0000000280000000-00000002bfffffff (prio 0, ram): alias 
virtio-mem-memslot-5 @mem0 0000000140000000-000000017fffffff
        00000002c0000000-00000002ffffffff (prio 0, ram): alias 
virtio-mem-memslot-6 @mem0 0000000180000000-00000001bfffffff
        0000000300000000-000000033fffffff (prio 0, ram): alias 
virtio-mem-memslot-7 @mem0 00000001c0000000-00000001ffffffff
        0000000340000000-000000037fffffff (prio 0, ram): alias 
virtio-mem-memslot-8 @mem0 0000000200000000-000000023fffffff
        0000000380000000-00000003bfffffff (prio 0, ram): alias 
virtio-mem-memslot-9 @mem0 0000000240000000-000000027fffffff
        00000003c0000000-00000003ffffffff (prio 0, ram): alias 
virtio-mem-memslot-10 @mem0 0000000280000000-00000002bfffffff
        0000000400000000-000000043fffffff (prio 0, ram): alias 
virtio-mem-memslot-11 @mem0 00000002c0000000-00000002ffffffff
        0000000440000000-000000047fffffff (prio 0, ram): alias 
virtio-mem-memslot-12 @mem0 0000000300000000-000000033fffffff
        0000000480000000-00000004bfffffff (prio 0, ram): alias 
virtio-mem-memslot-13 @mem0 0000000340000000-000000037fffffff
        00000004c0000000-00000004ffffffff (prio 0, ram): alias 
virtio-mem-memslot-14 @mem0 0000000380000000-00000003bfffffff
        0000000500000000-000000053fffffff (prio 0, ram): alias 
virtio-mem-memslot-15 @mem0 00000003c0000000-00000003ffffffff
        0000000540000000-000000057fffffff (prio 0, ram): alias 
virtio-mem-memslot-16 @mem0 0000000400000000-000000043fffffff
        0000000580000000-00000005bfffffff (prio 0, ram): alias 
virtio-mem-memslot-17 @mem0 0000000440000000-000000047fffffff
        00000005c0000000-00000005ffffffff (prio 0, ram): alias 
virtio-mem-memslot-18 @mem0 0000000480000000-00000004bfffffff
        0000000600000000-000000063fffffff (prio 0, ram): alias 
virtio-mem-memslot-19 @mem0 00000004c0000000-00000004ffffffff
        0000000640000000-000000067fffffff (prio 0, ram): alias 
virtio-mem-memslot-20 @mem0 0000000500000000-000000053fffffff

Requesting the VM to consume 5 GiB and rebooting (note: usable region size
will change during reboots) results in:
    0000000140000000-000001047fffffff (prio 0, i/o): device-memory
      0000000140000000-0000007e3fffffff (prio 0, i/o): virtio-mem-memslots
        0000000140000000-000000017fffffff (prio 0, ram): alias 
virtio-mem-memslot-0 @mem0 0000000000000000-000000003fffffff
        0000000180000000-00000001bfffffff (prio 0, ram): alias 
virtio-mem-memslot-1 @mem0 0000000040000000-000000007fffffff
        00000001c0000000-00000001ffffffff (prio 0, ram): alias 
virtio-mem-memslot-2 @mem0 0000000080000000-00000000bfffffff
        0000000200000000-000000023fffffff (prio 0, ram): alias 
virtio-mem-memslot-3 @mem0 00000000c0000000-00000000ffffffff
        0000000240000000-000000027fffffff (prio 0, ram): alias 
virtio-mem-memslot-4 @mem0 0000000100000000-000000013fffffff
        0000000280000000-00000002bfffffff (prio 0, ram): alias 
virtio-mem-memslot-5 @mem0 0000000140000000-000000017fffffff


In addition to other factors, we limit the number of memslots to 1024 per
devices and the size of one memslot to at least 1 GiB. So only a 1 TiB
virtio-mem device could consume 1024 memslots in the "worst" case. To
calculate a memslot limit for a device, we use a heuristic based on all
available memslots for memory devices and the percentage of
"device size":"total memory device area size". Users can further limit
the maximum number of memslots that will be used by a device by setting
the "max-memslots" property. It's expected to be set to "0" (auto) in most
setups.

In recent setups (e.g., KVM with ~32k memslots, vhost-user with ~4k
memslots after this series), we'll get the biggest benefit. In special
setups (e.g., older KVM, vhost kernel with 64 memslots), we'll get some
benefit -- the individual memslots will be bigger.

Future work:
- vhost-user and libvhost-user optimizations for handling more memslots
  more efficiently.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Ani Sinha <ani@anisinha.ca>
Cc: Peter Xu <peterx@redhat.com>
Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Philippe Mathieu-Daudé <f4bug@amsat.org>
Cc: kvm@vger.kernel.org

David Hildenbrand (15):
  memory: Drop mapping check from
    memory_region_get_ram_discard_manager()
  kvm: Return number of free memslots
  vhost: Return number of free memslots
  memory: Allow for marking memory region aliases unmergeable
  vhost: Don't merge unmergeable memory sections
  memory-device: Move memory_device_check_addable() directly into
    memory_device_pre_plug()
  memory-device: Generalize memory_device_used_region_size()
  memory-device: Support memory devices that consume a variable number
    of memslots
  vhost: Respect reserved memslots for memory devices when realizing a
    vhost device
  virtio-mem: Set the RamDiscardManager for the RAM memory region
    earlier
  virtio-mem: Fix typo in virito_mem_intersect_memory_section() function
    name
  virtio-mem: Expose device memory via separate memslots
  vhost-user: Increase VHOST_USER_MAX_RAM_SLOTS to 496 with
    CONFIG_VIRTIO_MEM
  libvhost-user: Increase VHOST_USER_MAX_RAM_SLOTS to 4096
  virtio-mem: Set "max-memslots" to 0 (auto) for the 6.2 machine

 accel/kvm/kvm-all.c                       |  24 ++-
 accel/stubs/kvm-stub.c                    |   4 +-
 hw/core/machine.c                         |   1 +
 hw/mem/memory-device.c                    | 167 +++++++++++++++---
 hw/virtio/vhost-stub.c                    |   2 +-
 hw/virtio/vhost-user.c                    |   7 +-
 hw/virtio/vhost.c                         |  17 +-
 hw/virtio/virtio-mem-pci.c                |  22 +++
 hw/virtio/virtio-mem.c                    | 202 ++++++++++++++++++++--
 include/exec/memory.h                     |  23 +++
 include/hw/mem/memory-device.h            |  32 ++++
 include/hw/virtio/vhost.h                 |   2 +-
 include/hw/virtio/virtio-mem.h            |  29 +++-
 include/sysemu/kvm.h                      |   2 +-
 softmmu/memory.c                          |  35 +++-
 stubs/qmp_memory_device.c                 |   5 +
 subprojects/libvhost-user/libvhost-user.h |   7 +-
 17 files changed, 499 insertions(+), 82 deletions(-)

-- 
2.31.1