[PATCH v2 00/16] virtio-mem: Expose device memory through multiple memslots

[David Hildenbrand]

Quoting from patch #14:

    Having large virtio-mem devices that only expose little memory to a VM
    is currently a problem: we map the whole sparse memory region into the
    guest using a single memslot, resulting in one gigantic memslot in KVM.
    KVM allocates metadata for the whole memslot, which can result in quite
    some memory waste.

    Assuming we have a 1 TiB virtio-mem device and only expose little (e.g.,
    1 GiB) memory, we would create a single 1 TiB memslot and KVM has to
    allocate metadata for that 1 TiB memslot: on x86, this implies allocating
    a significant amount of memory for metadata:

    (1) RMAP: 8 bytes per 4 KiB, 8 bytes per 2 MiB, 8 bytes per 1 GiB
        -> For 1 TiB: 2147483648 + 4194304 + 8192 = ~ 2 GiB (0.2 %)

        With the TDP MMU (cat /sys/module/kvm/parameters/tdp_mmu) this gets
        allocated lazily when required for nested VMs
    (2) gfn_track: 2 bytes per 4 KiB
        -> For 1 TiB: 536870912 = ~512 MiB (0.05 %)
    (3) lpage_info: 4 bytes per 2 MiB, 4 bytes per 1 GiB
        -> For 1 TiB: 2097152 + 4096 = ~2 MiB (0.0002 %)
    (4) 2x dirty bitmaps for tracking: 2x 1 bit per 4 KiB page
        -> For 1 TiB: 536870912 = 64 MiB (0.006 %)

    So we primarily care about (1) and (2). The bad thing is, that the
    memory consumption *doubles* once SMM is enabled, because we create the
    memslot once for !SMM and once for SMM.

    Having a 1 TiB memslot without the TDP MMU consumes around:
    * With SMM: 5 GiB
    * Without SMM: 2.5 GiB
    Having a 1 TiB memslot with the TDP MMU consumes around:
    * With SMM: 1 GiB
    * Without SMM: 512 MiB

    ... and that's really something we want to optimize, to be able to just
    start a VM with small boot memory (e.g., 4 GiB) and a virtio-mem device
    that can grow very large (e.g., 1 TiB).

    Consequently, using multiple memslots and only mapping the memslots we
    really need can significantly reduce memory waste and speed up
    memslot-related operations. Let's expose the sparse RAM memory region using
    multiple memslots, mapping only the memslots we currently need into our
    device memory region container.

The hyper-v balloon driver has similar demands [1].

For virtio-mem, this has to be turned manually on ("multiple-memslots=on"),
due to the interaction with vhost (below).

If we have less than 509 memslots available, we always default to a single
memslot. Otherwise, we automatically decide how many memslots to use
based on a simple heuristic (see patch #12), and try not to use more than
256 memslots across all memory devices: our historical DIMM limit.

As soon as any memory devices automatically decided on using more than
one memslot, vhost devices that support less than 509 memslots (e.g.,
currently most vhost-user devices like with virtiofsd) can no longer be
plugged as a precaution.

Quoting from patch #12:

    Plugging vhost devices with less than 509 memslots available while we
    have memory devices plugged that consume multiple memslots due to
    automatic decisions can be problematic. Most configurations might just fail
    due to "limit < used + reserved", however, it can also happen that these
    memory devices would suddenly consume memslots that would actually be
    required by other memslot consumers (boot, PCI BARs) later. Note that this
    has always been sketchy with vhost devices that support only a small number
    of memslots; but we don't want to make it any worse.So let's keep it simple
    and simply reject plugging such vhost devices in such a configuration.

    Eventually, all vhost devices that want to be fully compatible with such
    memory devices should support a decent number of memslots (>= 509).


The recommendation is to plug such vhost devices before the virtio-mem
decides, or to not set "multiple-memslots=on". As soon as these devices
support a reasonable number of memslots (>= 509), this will start working
automatically.

I run some tests on x86_64, now also including vfio tests. Seems to work
as expected, even when multiple memslots are used.


Patch #1 -- #3 are from [2] that were not picked up yet.

Patch #4 -- #12 add handling of multiple memslots to memory devices

Patch #13 -- #14 add "multiple-memslots=on" support to virtio-mem

Patch #15 -- #16 make sure that virtio-mem memslots can be enabled/disable
atomically


v1 -> v2:
* Include patches from [1]
* A lot of code simplification and reorganization, too many to spell out
* *don't* add a general soft-limit on memslots, to avoid warning in sane
  setups
* Simplify handling of vhost devices with a small number of memslots:
  simply fail plugging them
* "virtio-mem: Expose device memory via multiple memslots if enabled"
 -> Fix one "is this the last memslot" check
* Much more testing


[1] https://lkml.kernel.org/r/cover.1689786474.git.maciej.szmigiero@oracle.com
[2] https://lkml.kernel.org/r/20230523185915.540373-1-david@redhat.com

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Xiao Guangrong <xiaoguangrong.eric@gmail.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: "Philippe Mathieu-Daudé" <philmd@linaro.org>
Cc: Eduardo Habkost <eduardo@habkost.net>
Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Cc: Yanan Wang <wangyanan55@huawei.com>
Cc: Michal Privoznik <mprivozn@redhat.com>
Cc: Daniel P. Berrangé <berrange@redhat.com>
Cc: Gavin Shan <gshan@redhat.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Cc: kvm@vger.kernel.org

David Hildenbrand (16):
  vhost: Rework memslot filtering and fix "used_memslot" tracking
  vhost: Remove vhost_backend_can_merge() callback
  softmmu/physmem: Fixup qemu_ram_block_from_host() documentation
  kvm: Return number of free memslots
  vhost: Return number of free memslots
  memory-device: Support memory devices with multiple memslots
  stubs: Rename qmp_memory_device.c to memory_device.c
  memory-device: Track required and actually used memslots in
    DeviceMemoryState
  memory-device,vhost: Support memory devices that dynamically consume
    memslots
  kvm: Add stub for kvm_get_max_memslots()
  vhost: Add vhost_get_max_memslots()
  memory-device,vhost: Support automatic decision on the number of
    memslots
  memory: Clarify mapping requirements for RamDiscardManager
  virtio-mem: Expose device memory via multiple memslots if enabled
  memory,vhost: Allow for marking memory device memory regions
    unmergeable
  virtio-mem: Mark memslot alias memory regions unmergeable

 MAINTAINERS                                   |   1 +
 accel/kvm/kvm-all.c                           |  35 ++-
 accel/stubs/kvm-stub.c                        |   9 +-
 hw/mem/memory-device.c                        | 192 ++++++++++++-
 hw/virtio/vhost-stub.c                        |   9 +-
 hw/virtio/vhost-user.c                        |  21 +-
 hw/virtio/vhost-vdpa.c                        |   1 -
 hw/virtio/vhost.c                             | 103 +++++--
 hw/virtio/virtio-mem-pci.c                    |  21 ++
 hw/virtio/virtio-mem.c                        | 272 +++++++++++++++++-
 include/exec/cpu-common.h                     |  15 +
 include/exec/memory.h                         |  27 +-
 include/hw/boards.h                           |  14 +-
 include/hw/mem/memory-device.h                |  57 ++++
 include/hw/virtio/vhost-backend.h             |   9 +-
 include/hw/virtio/vhost.h                     |   3 +-
 include/hw/virtio/virtio-mem.h                |  23 +-
 include/sysemu/kvm.h                          |   4 +-
 include/sysemu/kvm_int.h                      |   1 +
 softmmu/memory.c                              |  35 ++-
 softmmu/physmem.c                             |  17 --
 .../{qmp_memory_device.c => memory_device.c}  |  10 +
 stubs/meson.build                             |   2 +-
 23 files changed, 775 insertions(+), 106 deletions(-)
 rename stubs/{qmp_memory_device.c => memory_device.c} (56%)

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2.41.0