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[Qemu-devel] [RFC] virtio-mem: paravirtualized memory

From: David Hildenbrand
Subject: [Qemu-devel] [RFC] virtio-mem: paravirtualized memory
Date: Fri, 16 Jun 2017 16:20:02 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.1.0


this is an idea that is based on Andrea Arcangeli's original idea to
host enforce guest access to memory given up using virtio-balloon using
userfaultfd in the hypervisor. While looking into the details, I
realized that host-enforcing virtio-balloon would result in way too many
problems (mainly backwards compatibility) and would also have some
conceptual restrictions that I want to avoid. So I developed the idea of
virtio-mem - "paravirtualized memory".

The basic idea is to add memory to the guest via a paravirtualized
mechanism (so the guest can hotplug it) and remove memory via a
mechanism similar to a balloon. This avoids having to online memory as
"online-movable" in the guest and allows more fain grained memory
hot(un)plug. In addition, migrating QEMU guests after adding/removing
memory gets a lot easier.

Actually, this has a lot in common with the XEN balloon or the Hyper-V
balloon (namely: paravirtualized hotplug and ballooning), but is very
different when going into the details.

Getting this all implemented properly will take quite some effort,
that's why I want to get some early feedback regarding the general
concept. If you have some alternative ideas, or ideas how to modify this
concept, I'll be happy to discuss. Just please make sure to have a look
at the requirements first.

0. Outline:
- I.    General concept
- II.   Use cases
- III.  Identified requirements
- IV.   Possible modifications
- V.    Prototype
- VI.   Problems to solve / things to sort out / missing in prototype
- VII.  Questions

I. General concept

We expose memory regions to the guest via a paravirtualize interface. So
instead of e.g. a DIMM on x86, such memory is not anounced via ACPI.
Unmodified guests (without a virtio-mem driver) won't be able to see/use
this memory. The virtio-mem guest driver is needed to detect and manage
these memory areas. What makes this memory special is that it can grow
while the guest is running ("plug memory") and might shrink on a reboot
(to compensate "unplugged" memory - see next paragraph). Each virtio-mem
device manages exactly one such memory area. By having multiple ones
assigned to different NUMA nodes, we can modify memory on a NUMA basis.

Of course, we cannot shrink these memory areas while the guest is
running. To be able to unplug memory, we do something like a balloon
does, however limited to this very memory area that belongs to the
virtio-mem device. The guest will hand back small chunks of memory. If
we want to add memory to the guest, we first "replug" memory that has
previously been given up by the guest, before we grow our memory area.

On a reboot, we want to avoid any memory holes in our memory, therefore
we resize our memory area (shrink it) to compensate memory that has been
unplugged. This highly simplifies hotplugging memory in the guest (
hotplugging memory with random memory holes is basically impossible).

We have to make sure that all memory chunks the guest hands back on
unplug requests will not consume memory in the host. We do this by
write-protecting that memory chunk in the host and then dropping the
backing pages. The guest can read this memory (reading from the ZERO
page) but no longer write to it. For now, this will only work on
anonymous memory. We will use userfaultfd WP (write-protect mode) to
avoid creating too many VMAs. Huge pages will require more effort (no
explicit ZERO page).

As we unplug memory on a fine grained basis (and e.g. not on
a complete DIMM basis), there is no need to online virtio-mem memory
as online-movable. Also, memory unplug support for Windows might be
supported that way. You can find more details in the Q/A section below.

The important points here are:
- After a reboot, every memory the guest sees can be accessed and used.
  (in contrast to e.g. the XEN balloon, see Q/A fore more details)
- Rebooting into an unmodified guest will not result into random
  crashed. The guest will simply not be able to use all memory without a
  virtio-mem driver.
- Adding/Removing memory will not require modifying the QEMU command
  line on the migration target. Migration simply works (re-sizing memory
  areas is already part of the migration protocol!). Essentially, this
  makes adding/removing memory to/from a guest way simpler and
  independent of the underlying architecture. If the guest OS can online
  new memory, we can add more memory this way.
- Unplugged memory can be read. This allows e.g. kexec() without nasty
  modifications. Especially relevant for Windows' kexec() variant.
- It will play nicely with other things mapped into the address space,
  e.g. also other DIMMs or NVDIMM. virtio-mem will only work on its own
  memory region (in contrast e.g. to virtio-balloon). Especially it will
  not give up ("allocate") memory on other DIMMs, hindering them to get
  unplugged the ACPI way.
- We can add/remove memory without running into KVM memory slot or other
  (e.g. ACPI slot) restrictions. The granularity in which we can add
  memory is only limited by the granularity the guest can add memory
  (e.g. Windows 2MB, Linux on x86 128MB for now).
- By not having to online memory as online-movable we don't run into any
  memory restrictions in the guest. E.g. page tables can only be created
  on !movable memory. So while there might be plenty of online-movable
  memory left, allocation of page tables might fail. See Q/A for more
- The admin will not have to set memory offline in the guest first in
  order to unplug it. virtio-mem will handle this internally and not
  require interaction with an admin or a guest-agent.

Important restrictions of this concept:
- Guests without a virtio-mem guest driver can't see that memory.
- We will always require some boot memory that cannot get unplugged.
  Also, virtio-mem memory (as all other hotplugged memory) cannot become
  DMA memory under Linux. So the boot memory also defines the amount of
  DMA memory.
- Hibernation/Sleep+Restore while virtio-mem is active is not supported.
  On a reboot/fresh start, the size of the virtio-mem memory area might
  change and a running/loaded guest can't deal with that.
- Unplug support for hugetlbfs/shmem will take quite some time to
  support. The larger the used page size, the harder for the guest to
  give up memory. We can still use DIMM based hotplug for that.
- Huge huge pages are problematic, as the guest would have to give up
  e.g. 1GB chunks. This is not expected to be supported. We can still
  use DIMM based hotplug for setups that require that.
- For any memory we unplug using this mechanism, for now we will still
  have struct pages allocated in the guest. This means, that roughly
  1.6% of unplugged memory will still be allocated in the guest, being

II. Use cases

Of course, we want to deny any access to unplugged memory. In contrast
to virtio-balloon or other similar ideas (free page hinting), this is
not about cooperative memory management, but about guarantees. The idea
is, that both concepts can coexist.

So one use case is of course cloud providers. Customers can add
or remove memory to/from a VM without having to care about how to
online memory or in which amount to add memory in the first place in
order to remove it again. In cloud environments, we care about
guarantees. E.g. for virtio-balloon a malicious guest can simply reuse
any deflated memory, and the hypervisor can't even tell if the guest is
malicious (e.g. a harmless guest reboot might look like a malicious
guest). For virtio-mem, we guarantee that the guest can't reuse any
memory that it previously gave up.

But also for ordinary VMs (!cloud), this avoids having to online memory
in the guest as online-movable and therefore not running into allocation
problems if there are e.g. many processes needing many page tables on
!movable memory. Also here, we don't have to know how much memory we
want to remove some-when in the future before we add memory. (e.g. if we
add a 128GB DIMM, we can only remove that 128GB DIMM - if we are lucky).

We might be able to support memory unplug for Windows (as for now,
ACPI unplug is not supported), more details have to be clarified.

As we can grow these memory areas quite easily, another use case might
be guests that tell us they need more memory. Thinking about VMs to
protect containers, there seems to be the general problem that we don't
know how much memory the container will actually need. We could
implement a mechanism (in virtio-mem or guest driver), by which the
guest can request more memory. If the hypervisor agrees, it can simply
give the guest more memory. As this is all handled within QEMU,
migration is not a problem. Adding more memory will not result in new
DIMM devices.

III. Identified requirements

I considered the following requirements.

NUMA aware:
  We want to be able to add/remove memory to/from NUMA nodes.
Different page-size support:
  We want to be able to support different page sizes, e.g. because of
  huge pages in the hypervisor or because host and guest have different
  page sizes (powerpc 64k vs 4k).
  There has to be no way the guest can reuse unplugged memory without
  host consent. Still, we could implement a mechanism for the guest to
  request more memory. The hypervisor then has to decide how it wants to
  handle that request.
Architecture independence:
  We want this to work independently of other technologies bound to
  specific architectures, like ACPI.
Avoid online-movable:
  We don't want to have to online memory in the guest as online-movable
  just to be able to unplug (at least parts of) it again.
Migration support:
  Be able to migrate without too much hassle. Especially, to handle it
  completely within QEMU (not having to add new devices to the target
  command line).
Windows support:
  We definitely want to support Windows guests in the long run.
Coexistence with other hotplug mechanisms:
  Allow to hotplug DIMMs / NVDIMMs, therefore to share the "hotplug"
  address space part with other devices.
Backwards compatibility:
  Don't break if rebooting into an unmodified guest after having
  unplugged some memory. All memory a freshly booted guest sees must not
  contain memory holes that will crash it if it tries to access it.

IV. Possible modifications

Adding a guest->host request mechanism would make sense to e.g. be able
to request further memory from the hypervisor directly from the guest.

Adding memory will be much easier than removing memory. We can split
this up and first introduce "adding memory" and later add "removing
memory". Removing memory will require userfaultfd WP in the hypervisor
and a special fancy allocator in the guest. So this will take some time.

Adding a mechanism to trade in memory blocks might make sense to allow
some sort of memory compaction. However I expect this to be highly
complicated and basically not feasible.

Being able to unplug memory "any" memory instead of only memory
belonging to the virtio-mem device sounds tempting (and simplifies
certain parts), however it has a couple of side effects I want to avoid.
You can read more about that in the Q/A below.

V. Prototype

To identify potential problems I developed a very basic prototype. It
is incomplete, full of hacks and most probably broken in various ways.
I used it only in the given setup, only on x86 and only with an initrd.

It uses a fixed page size of 256k for now, has a very ugly allocator
hack in the guest, the virtio protocol really needs some tuning and
an async job interface towards the user is missing. Instead of using
userfaultfd WP, I am using simply mprotect() in this prototype. Basic
migration works (not involving userfaultfd).

Please, don't even try to review it (that's why I will also not attach
any patches to this mail :) ), just use this as an inspiration what this
could look like. You can find the latest hack at:

QEMU: https://github.com/davidhildenbrand/qemu/tree/virtio-mem

Kernel: https://github.com/davidhildenbrand/linux/tree/virtio-mem

Use the kernel in the guest and make sure to compile the virtio-mem
driver into the kernel (CONFIG_VIRTIO_MEM=y). A host kernel patch is
contained to allow atomic resize of KVM memory regions, however it is
pretty much untested.

1. Starting a guest with virtio-mem memory:
   We will create a guest with 2 NUMA nodes and 4GB of "boot + DMA"
   memory. This memory is visible also to guests without virtio-mem.
   Also, we will add 4GB to NUMA node 0 and 3GB to NUMA node 1 using
   virtio-mem. We allow both virtio-mem devices to grow up to 8GB. The
   last 4 lines are the important part.

--> qemu/x86_64-softmmu/qemu-system-x86_64 \
        -m 4G,maxmem=20G \
        -smp sockets=2,cores=2 \
        -numa node,nodeid=0,cpus=0-1 -numa node,nodeid=1,cpus=2-3 \
        -machine pc \
        -kernel linux/arch/x86_64/boot/bzImage \
        -nodefaults \
        -chardev stdio,id=serial \
        -device isa-serial,chardev=serial \
        -append "console=ttyS0 rd.shell rd.luks=0 rd.lvm=0" \
        -initrd /boot/initramfs-4.10.8-200.fc25.x86_64.img \
        -chardev socket,id=monitor,path=/var/tmp/monitor,server,nowait \
        -mon chardev=monitor,mode=readline \
        -object memory-backend-ram,id=mem0,size=4G,max-size=8G \
        -device virtio-mem-pci,id=reg0,memdev=mem0,node=0 \
        -object memory-backend-ram,id=mem1,size=3G,max-size=8G \
        -device virtio-mem-pci,id=reg1,memdev=mem1,node=1

2. Listing current memory assignment:

--> (qemu) info memory-devices
        Memory device [virtio-mem]: "reg0"
          addr: 0x140000000
          node: 0
          size: 4294967296
          max-size: 8589934592
          memdev: /objects/mem0
        Memory device [virtio-mem]: "reg1"
          addr: 0x340000000
          node: 1
          size: 3221225472
          max-size: 8589934592
          memdev: /objects/mem1
--> (qemu) info numa
        2 nodes
        node 0 cpus: 0 1
        node 0 size: 6144 MB
        node 1 cpus: 2 3
        node 1 size: 5120 MB

3. Resize a virtio-mem device: Unplugging memory.
   Setting reg0 to 2G (remove 2G from NUMA node 0)

--> (qemu) virtio-mem reg0 2048
        virtio-mem reg0 2048
--> (qemu) info numa
        info numa
        2 nodes
        node 0 cpus: 0 1
        node 0 size: 4096 MB
        node 1 cpus: 2 3
        node 1 size: 5120 MB

4. Resize a virtio-mem device: Plugging memory
   Setting reg0 to 8G (adding 6G to NUMA node 0) will replug 2G and plug
   4G, automatically re-sizing the memory area. You might experience
   random crashes at this point if the host kernel missed a KVM patch
   (as the memory slot is not re-sized in an atomic fashion).

--> (qemu) virtio-mem reg0 8192
        virtio-mem reg0 8192
--> (qemu) info numa
        info numa
        2 nodes
        node 0 cpus: 0 1
        node 0 size: 10240 MB
        node 1 cpus: 2 3
        node 1 size: 5120 MB

5. Resize a virtio-mem device: Try to unplug all memory.
   Setting reg0 to 0G (removing 8G from NUMA node 0) will not work. The
   guest will not be able to unplug all memory. In my example, 164M
   cannot be unplugged (out of memory).

--> (qemu) virtio-mem reg0 0
        virtio-mem reg0 0
--> (qemu) info numa
        info numa
        2 nodes
        node 0 cpus: 0 1
        node 0 size: 2212 MB
        node 1 cpus: 2 3
        node 1 size: 5120 MB
--> (qemu) info virtio-mem reg0
        info virtio-mem reg0
        Status: ready
        Request status: vm-oom
        Page size: 2097152 bytes
--> (qemu) info memory-devices
        Memory device [virtio-mem]: "reg0"
          addr: 0x140000000
          node: 0
          size: 171966464
          max-size: 8589934592
          memdev: /objects/mem0
        Memory device [virtio-mem]: "reg1"
          addr: 0x340000000
          node: 1
          size: 3221225472
          max-size: 8589934592
          memdev: /objects/mem1

At any point, we can migrate our guest without having to care about
modifying the QEMU command line on the target side. Simply start the
target e.g. with an additional '-incoming "exec: cat IMAGE"' and you're

VI. Problems to solve / things to sort out / missing in prototype

- We need an async job API to send the unplug/replug/plug requests to
  the guest and query the state. [medium/hard]
- Handle various alignment problems. [medium]
- We need a virtio spec

Relevant for plug:
- Resize QEMU memory regions while the guest is running (esp. grow).
  While I implemented a demo solution for KVM memory slots, something
  similar would be needed for vhost. Re-sizing of memory slots has to be
  an atomic operation. [medium]
- NUMA: Most probably the NUMA node should not be part of the virtio-mem
  device, this should rather be indicated via e.g. ACPI. [medium]
- x86: Add the complete possible memory to the a820 map as reserved.
- x86/powerpc/...: Indicate to which NUMA node the memory belongs using
  ACPI. [medium]
- x86/powerpc/...: Share address space with ordinary DIMMS/NVDIMMs, for
  now this is blocked for simplicity. [medium/hard]
- If the bitmaps become too big, migrate them like memory. [medium]

Relevant for unplug:
- Allocate memory in Linux from a specific memory range. Windows has a
  nice interface for that (at least it looks nice when reading the API).
  This could be done using fake NUMA nodes or a new ZONE. My prototype
  just uses a very ugly hack. [very hard]
- Use userfaultfd WP (write-protect) insted of mprotect. Especially,
  have multiple userfaultfd user in QEMU at a time (postcopy).

Stuff for the future:
- Huge pages are problematic (no ZERO page support). This might not be
  trivial to support. [hard/very hard]
- Try to free struct pages, to avoid the 1.6% overhead [very very hard]

VII. Questions

To get unplug working properly, it will require quite some effort,
that's why I want to get some basic feedback before continuing working
on a RFC implementation + RFC virtio spec.

a) Did I miss anything important? Are there any ultimate blockers that I
   ignored? Any concepts that are broken?

b) Are there any alternatives? Any modifications that could make life
   easier while still taking care of the requirements?

c) Are there other use cases we should care about and focus on?

d) Am I missing any requirements? What else could be important for
   !cloud and cloud?

e) Are there any possible solutions to the allocator problem (allocating
   memory from a specific memory area)? Please speak up!

f) Anything unclear?

e) Any feelings about this? Yay or nay?

As you reached this point: Thanks for having a look!!! Highly appreciated!


Q: What's the problem with ordinary memory hot(un)plug?

A: 1. We can only unplug in the granularity we plugged. So we have to
      know in advance, how much memory we want to remove later on. If we
      plug a 2G dimm, we can only unplug a 2G dimm.
   2. We might run out of memory slots. Although very unlikely, this
      would strike if we try to always plug small modules in order to be
      able to unplug again (e.g. loads of 128MB modules).
   3. Any locked page in the guest can hinder us from unplugging a dimm.
      Even if memory was onlined as online_movable, a single locked page
      can hinder us from unplugging that memory dimm.
   4. Memory has to be onlined as online_movable. If we don't put that
      memory into the movable zone, any non-movable kernel allocation
      could end up on it, turning the complete dimm unpluggable. As
      certain allocations cannot go into the movable zone (e.g. page
      tables), the ratio between online_movable/online memory depends on
      the workload in the guest. Ratios of 50% -70% are usually fine.
      But it could happen, that there is plenty of memory available,
      but kernel allocations fail. (source: Andrea Arcangeli)
   5. Unplugging might require several attempts. It takes some time to
      migrate all memory from the dimm. At that point, it is then not
      really obvious why it failed, and whether it could ever succeed.
   6. Windows does support memory hotplug but not memory hotunplug. So
      this could be a way to support it also for Windows.
Q: Will this work with Windows?

A: Most probably not in the current form. Memory has to be at least
   added to the a820 map and ACPI (NUMA). Hyper-V ballon is also able to
   hotadd memory using a paravirtualized interface, so there are very
   good chances that this will work. But we won't know for sure until we
   also start prototyping.
Q: How does this compare to virtio-balloo?

A: In contrast to virtio-balloon, virtio-mem
   1. Supports multiple page sizes, even different ones for different
      virtio-mem devices in a guest.
   2. Is NUMA aware.
   3. Is able to add more memory.
   4. Doesn't work on all memory, but only on the managed one.
   5. Has guarantees. There is now way for the guest to reclaim memory.
Q: How does this compare to XEN balloon?

A: XEN balloon also has a way to hotplug new memory. However, on a
   reboot, the guest will "see" more memory than it actually has.
   Compared to XEN balloon, virtio-mem:
   1. Supports multiple page sizes.
   2. Is NUMA aware.
   3. The guest can survive a reboot into a system without the guest
      driver. If the XEN guest driver doesn't come up, the guest will
      get killed once it touches too much memory.
   4. Reboots don't require any hacks.
   5. The guest knows which memory is special. And it remains special
      during a reboot. Hotplugged memory not suddenly becomes base
      memory. The balloon mechanism will only work on a specific memory
Q: How does this compare to Hyper-V balloon?

A: Based on the code from the Linux Hyper-V balloon driver, I can say
   that Hyper-V also has a way to hotplug new memory. However, memory
   will remain plugged on a reboot. Therefore, the guest will see more
   memory than the hypervisor actually wants to assign to it.
   Virtio-mem in contrast:
   1. Supports multiple page sizes.
   2. Is NUMA aware.
   3. I have no idea what happens under Hyper-v when
      a) rebooting into a guest without a fitting guest driver
      b) kexec() touches all memory
      c) the guest misbehaves
   4. The guest knows which memory is special. And it remains special
      during a reboot. Hotpplugged memory not suddenly becomes base
      memory. The balloon mechanism will only work on a specific memory
   In general, it looks like the hypervisor has to deal with malicious
   guests trying to access more memory than desired by providing enough
   swap space.
Q: How is virtio-mem NUMA aware?

A: Each virtio-mem device belongs exactly to one NUMA node (if NUMA is
   enabled). As we can resize these regions separately, we can control
   from/to which node to remove/add memory.
Q: Why do we need support for multiple page sizes?

A: If huge pages are used in the host, we can only guarantee that they
   are not accessible by the guest anymore, if the guest gives up memory
   in this granularity. We prepare for that. Also, powerpc can have 64k
   pages in the host but 4k pages in the guest. So the guest must only
   give up 64k chunks. In addition, unplugging 4k pages might be bad
   when it comes to fragmentation. My prototype currently uses 256k. We
   can make this configurable - and it can vary for each virtio-mem
Q: What are the limitations with paravirtualized memory hotplug?

A: The same as for DIMM based hotplug, but we don't run out of any
   memory/ACPI slots. E.g. on x86 Linux, only 128MB chunks can be
   hotplugged, on x86 Windows it's 2MB. In addition, of course we
   have to take care of maximum address limits in the guest. The idea
   is to communicate these limits to the hypervisor via virtio-mem,
   to give hints when trying to add/remove memory.
Q: Why not simply unplug *any* memory like virtio-balloon does?

A: This could be done and a previous prototype did it like that.
   However, there are some points to consider here.
   1. If we combine this with ordinary memory hotplug (DIMM), we most
      likely won't be able to unplug DIMMs anymore as virtio-mem memory
      gets "allocated" on these.
   2. All guests using virtio-mem cannot use huge pages as backing
      storage at all (as virtio-mem only supports anonymous pages).
   3. We need to track unplugged memory for the complete address space,
      so we need a global state in QEMU. Bitmaps get bigger. We will not
      be abe to dynamically grow the bitmaps for a virtio-mem device.
   4. Resolving/checking memory to be unplugged gets significantly
      harder. How should the guest know which memory it can unplug for a
      specific virtio-mem device? E.g. if NUMA is active, only that NUMA
      node to which a virtio-mem device belongs can be used.
   5. We will need userfaultfd handler for the complete address space,
      not just for the virtio-mem managed memory.
      Especially, if somebody hotplugs a DIMM, we dynamically will have
      to enable the userfaultfd handler.
   6. What shall we do if somebody hotplugs a DIMM with huge pages? How
      should we tell the guest, that this memory cannot be used for
   In summary: This concept is way cleaner, but also harder to
Q: Why not reuse virtio-balloon?

A: virtio-balloon is for cooperative memory management. It has a fixed
   page size and will deflate in certain situations. Any change we
   introduce will break backwards compatibility. virtio-balloon was not
   designed to give guarantees. Nobody can hinder the guest from
   deflating/reusing inflated memory. In addition, it might make perfect
   sense to have both, virtio-balloon and virtio-mem at the same time,
   especially looking at the DEFLATE_ON_OOM or STATS features of
   virtio-balloon. While virtio-mem is all about guarantees, virtio-
   balloon is about cooperation.
Q: Why not reuse acpi hotplug?

A: We can easily run out of slots, migration in QEMU will just be
   horrible and we don't want to bind virtio* to architecture specific
   E.g. thinking about s390x - no ACPI. Also, mixing an ACPI driver with
   a virtio-driver sounds very weird. If the virtio-driver performs the
   hotplug itself, we might later perform some extra tricks: e.g.
   actually unplug certain regions to give up some struct pages.

   We want to manage the way memory is added/removed completely in QEMU.
   We cannot simply add new device from within QEMU and expect that
   migration in QEMU will work.
Q: Why do we need resizable memory regions?

A: Migration in QEMU is special. Any device we have on our source VM has
   to already be around on our target VM. So simply creating random
   devides internally in QEMU is not going to work. The concept of
   resizable memory regions in QEMU already exists and is part of the
   migration protocol. Before memory is migrated, the memory is resized.
   So in essence, this makes migration support _a lot_ easier.

   In addition, we won't run in any slot number restriction when
   automatically managing how to add memory in QEMU.
Q: Why do we have to resize memory regions on a reboot?

A: We have to compensate all memory that has been unplugged for that
   area by shrinking it, so that a fresh guest can use all memory when
   initializing the virtio-mem device.
Q: Why do we need userfaultfd?

A: mprotect() will create a lot of VMAs in the kernel. This will degrade
   performance and might even fail at one point. userfaultfd avoids this
   by not creating a new VMA for every protected range. userfaultfd WP
   is currently still under development and suffers from false positives
   that make it currently impossible to properly integrate this into the
Q: Why do we have to allow reading unplugged memory?

A: E.g. if the guest crashes and want's to write a memory dump, it will
   blindly access all memory. While we could find ways to fixup kexec,
   Windows dumps might be more problematic. Allowing the guest to read
   all memory (resulting in reading all 0's) safes us from a lot of

   The downside is, that page tables full of zero pages might be
   created. (we might be able to find ways to optimize this)
Q: Will this work with postcopy live-migration?

A: Not in the current form. And it doesn't really make sense to spend
   time on it as long as we don't use userfaultfd. Combining both
   handlers will be interesting. It can be done with some effort on the
   QEMU side.
Q: What's the problem with shmem/hugetlbfs?

A: We currently rely on the ZERO page to be mapped when the guest tries
   to read unplugged memory. For shmem/hugetlbfs, there is no ZERO page,
   so read access would result in memory getting populated. We could
   either introduce an explicit ZERO page, or manage it using one dummy
   ZERO page (using regular usefaultfd, allow only one such page to be
   mapped at a time). For now, only anonymous memory.
Q: Ripping out random page ranges, won't this fragment our guest memory?

A: Yes, but depending on the virtio-mem page size, this might be more or
   less problematic. The smaller the virtio-mem page size, the more we
   fragment and make small allocations fail. The bigger the virtio-mem
   page size, the higher the chance that we can't unplug any more
Q: Why can't we use memory compaction like virtio-balloon?

A: If the virtio-mem page size > PAGE_SIZE, we can't do ordinary
   page migration, migration would have to be done in blocks. We could
   later add an guest->host virtqueue, via which the guest can
   "exchange" memory ranges. However, also mm has to support this kind
   of migration. So it is not completely out of scope, but will require
   quite some work.
Q: Do we really need yet another paravirtualized interface for this?

A: You tell me :)



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