Re: [PATCH v1 00/12] virtio-mem: Expose device memory via multiple memslots

[Michael S. Tsirkin]

On Wed, Oct 27, 2021 at 02:45:19PM +0200, David Hildenbrand wrote:
> This is the follow-up of [1], dropping auto-detection and vhost-user
> changes from the initial RFC.
> 
> 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.

I'm confused by the mention of these security considerations,
and I expect users will be just as confused.
So let's say user wants to not be exposed. What value for
the option should be used? What if a lower option is used?
Is there still some security advantage?

> 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.
> 
> The user configures how many memslots a virtio-mem device should use, the
> default is "1" -- essentially corresponding to the old behavior.
> 
> 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 with "memslots=500" 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 (e.g., device block size), we limit the number
> of memslots to 1024 per devices and the size of one memslot to at least
> 128 MiB. Further, we make sure internally to align the memslot size to at
> least 128 MiB. For now, we limit the total number of memslots that can
> be used by memory devices to 2048, to no go crazy on individual RAM
> mappings in our address spaces.
> 
> Future work:
> - vhost-user and libvhost-user/vhost-user-backend changes to support more than
>   32 memslots.
> - "memslots=0" mode to allow for auto-determining the number of memslots to
>   use.
> - Eventually have an interface to query the memslot limit for a QEMU
>   instance. But vhost-* devices complicate that matter.
> 
> RCF -> v1:
> - Dropped "max-memslots=" parameter and converted to "memslots=" parameter
> - Dropped auto-determining the number of memslots to use
> - Dropped vhost* memslot changes
> - Improved error messages regarding memory slot limits
> - Reshuffled, cleaned up patches, rewrote patch descriptions
> 
> 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: Hui Zhu <teawater@gmail.com>
> Cc: Sebastien Boeuf <sebastien.boeuf@intel.com>
> Cc: kvm@vger.kernel.org
> 
> [1] https://lkml.kernel.org/r/20211013103330.26869-1-david@redhat.com
> 
> David Hildenbrand (12):
>   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 dynamically consume
>     multiple memslots
>   vhost: Respect reserved memslots for memory devices when realizing a
>     vhost device
>   memory: Drop mapping check from
>     memory_region_get_ram_discard_manager()
>   virtio-mem: Fix typo in virito_mem_intersect_memory_section() function
>     name
>   virtio-mem: Set the RamDiscardManager for the RAM memory region
>     earlier
>   virtio-mem: Expose device memory via multiple memslots
> 
>  accel/kvm/kvm-all.c            |  24 ++--
>  accel/stubs/kvm-stub.c         |   4 +-
>  hw/mem/memory-device.c         | 115 ++++++++++++++----
>  hw/virtio/vhost-stub.c         |   2 +-
>  hw/virtio/vhost.c              |  21 ++--
>  hw/virtio/virtio-mem-pci.c     |  23 ++++
>  hw/virtio/virtio-mem.c         | 212 +++++++++++++++++++++++++++++----
>  include/exec/memory.h          |  23 ++++
>  include/hw/mem/memory-device.h |  33 +++++
>  include/hw/virtio/vhost.h      |   2 +-
>  include/hw/virtio/virtio-mem.h |  25 +++-
>  include/sysemu/kvm.h           |   2 +-
>  softmmu/memory.c               |  35 ++++--
>  stubs/qmp_memory_device.c      |   5 +
>  14 files changed, 449 insertions(+), 77 deletions(-)
> 
> -- 
> 2.31.1