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+# Memory Hotplugging with virtio-mem
+
+## What is virtio-mem
+
+`virtio-mem` is a para-virtualized memory device that enables dynamic memory
+resizing for virtual machines. Unlike traditional memory hot-plug mechanisms,
+`virtio-mem` provides a flexible and efficient solution that works across
+different architectures and avoids many limitations of older approaches.
+
+The `virtio-mem` device manages a contiguous memory region that is divided into
+fixed-size blocks. The host can request the guest to plug (make available) or
+unplug (release) memory by changing the device's target size, and the guest
+driver responds by allocating or freeing memory blocks accordingly. This
+approach provides fine-grained control over guest memory with minimal overhead.
+
+Firecracker further adds the concept of slots, which are a set of contiguous
+blocks (usually 128MiB) that can be fully protected from guest accesses to
+prevent malicious guests from accessing the hotpluggable memory range when not
+allowed by the host.
+
+## Prerequisites
+
+To support memory hotplugging via `virtio-mem`, you must use a guest kernel with
+the appropriate version and configuration options enabled as follows:
+
+#### Kernel Version
+
+For x86_64, Firecracker requires a kernel version >=5.16 as it requires the
+guest driver to negotiate `VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE`. For aarch64, a
+kernel >=5.18 is required. Refer to the
+[kernel policy documentation](kernel-policy.md) for a list of officially
+supported guest kernels.
+
+#### Kernel Config
+
+The following kernel configuration options are required:
+
+- `CONFIG_VIRTIO_MEM=y`: Enables the `virtio-mem` driver
+- `CONFIG_MEMORY_HOTPLUG=y`: Enables memory hotplug support
+- `CONFIG_MEMORY_HOTREMOVE=y`: Enables memory hot-remove support (recommended)
+- `CONFIG_STRICT_DEVMEM`: (aarch64 only) required to enable the `virtio-mem`
+  driver.
+
+## Adding hotpluggable memory
+
+The `virtio-mem` device must be configured during VM setup with the total amount
+of memory that can be hotplugged, before starting the virtual machine. This can
+be done through a `PUT` request on `/hotplug/memory` or by including the
+configuration in the JSON configuration file.
+
+### Configuration Parameters
+
+- `total_size_mib` (required): The maximum size of hotpluggable memory in MiB.
+  This defines the upper bound of memory that can be added to the VM. Must be a
+  multiple of `slot_size_mib`.
+
+- `block_size_mib` (optional, default: 2): The size of individual memory blocks
+  in MiB. Must be at least 2 MiB and a power of 2. Larger block sizes provide
+  better performance but less granularity (harder for the guest to unplug).
+
+- `slot_size_mib` (optional, default: 128): The size of KVM memory slots in MiB.
+  Must be at least `block_size_mib` and a power of 2. Larger slot sizes improve
+  performance for large memory operations but reduce unplugging protection
+  efficiency.
+
+We recommend leaving these values to the default unless strict memory protection
+is required, in which case `block_size_mib` should be equal to `slot_size_mib`.
+Note that this will make it harder for the guest kernel to find contiguous
+memory to hot-un-plug. Refer to the [Memory Protection](#memory-protection)
+section below for more details.
+
+### API Configuration
+
+Here is an example of how to configure the `virtio-mem` device via the API:
+
+```console
+socket_location=/run/firecracker.socket
+
+curl --unix-socket $socket_location -i \
+    -X PUT 'http://localhost/hotplug/memory' \
+    -H 'Accept: application/json' \
+    -H 'Content-Type: application/json' \
+    -d "{
+        \"total_size_mib\": 1024,
+        \"block_size_mib\": 2,
+        \"slot_size_mib\": 128
+    }"
+```
+
+Note that this is only allowed before the `InstanceStart` action and not on
+snapshot-restored VMs (which will use the configuration saved in the snapshot).
+
+### JSON Configuration
+
+To configure via JSON, add the following to your VM configuration file:
+
+```json
+{
+    "memory-hotplug": {
+        "total_size_mib": 1024,
+        "block_size_mib": 2,
+        "slot_size_mib": 128
+    }
+}
+```
+
+### Checking Device Status
+
+After configuration, you can query the device status at any time:
+
+```console
+socket_location=/run/firecracker.socket
+
+curl --unix-socket $socket_location -i \
+    -X GET 'http://localhost/hotplug/memory' \
+    -H 'Accept: application/json'
+```
+
+This returns information about the current device state, including:
+
+- `total_size_mib`: Maximum hotpluggable memory size
+- `block_size_mib`: Block size used by the device
+- `slot_size_mib`: KVM slot size
+- `plugged_size_mib`: Currently plugged (available) memory by the guest
+- `requested_size_mib`: Target memory size set by the host
+
+## Operating the virtio-mem device
+
+Once configured and the VM is running, you can dynamically adjust the amount of
+memory available to the guest by updating the requested size.
+
+### Hot-plugging Memory
+
+To add memory to a running VM, request a greater size from the `virtio-mem`
+device:
+
+```console
+socket_location=/run/firecracker.socket
+
+curl --unix-socket $socket_location -i \
+    -X PATCH 'http://localhost/hotplug/memory' \
+    -H 'Accept: application/json' \
+    -H 'Content-Type: application/json' \
+    -d "{
+        \"requested_size_mib\": 512
+    }"
+```
+
+This updates the target memory size. The guest driver will detect this change
+and allocate memory blocks to reach the requested size. The process is
+asynchronous - the guest will incrementally plug memory until it reaches the
+target. It is recommended to use the GET API to monitor the current state of the
+hot-plugging by the driver.
+
+### Hot-removing Memory
+
+To remove memory from a running VM, request a lower size:
+
+```console
+socket_location=/run/firecracker.socket
+
+curl --unix-socket $socket_location -i \
+    -X PATCH 'http://localhost/hotplug/memory' \
+    -H 'Accept: application/json' \
+    -H 'Content-Type: application/json' \
+    -d "{
+        \"requested_size_mib\": 256
+    }"
+```
+
+Setting a lower `requested_size_mib` value causes the guest driver to free
+memory blocks. Once the guest reports a block to be unplugged, the unplugged
+memory is immediately freed from the host process. If all blocks in a memory
+slot are unplugged, then Firecracker will also protect the memory slot, removing
+access from the guest.
+
+To remove all hotplugged memory, set `requested_size_mib` to 0:
+
+```console
+curl --unix-socket $socket_location -i \
+    -X PATCH 'http://localhost/hotplug/memory' \
+    -H 'Accept: application/json' \
+    -H 'Content-Type: application/json' \
+    -d '{"requested_size_mib": 0}'
+```
+
+Note that this requires the guest to actually be able to find and report memory
+blocks that can be moved or freed.
+
+## Configuring the guest driver
+
+The guest kernel must be configured with specific boot or runtime module
+parameters to ensure optimal behavior of the `virtio-mem` driver and memory
+hotplug module.
+
+In short:
+
+- pass `memhp_default_state=online_movable` if hot-removal is required and the
+  hotpluggable memory area is not much bigger than the normal memory.
+- pass `memory_hotplug.memmap_on_memory=1 memhp_default_state=online` if
+  hot-removal is not required and the hotpluggable memory area can be much
+  bigger than the normal memory.
+
+#### `memhp_default_state`
+
+This parameter controls how newly hotplugged memory is onlined by the kernel.
+This parameter is required for automatically onlining new memory pages. It is
+recommended to set it to `online_movable` as below for reliable memory
+hot-removal.
+
+```
+memhp_default_state=online_movable
+```
+
+The `online_movable` setting ensures that:
+
+- Hotplugged memory is placed in the MOVABLE zone
+- The kernel can migrate pages when unplugging is requested
+- Memory can be successfully freed back to the host
+
+Other possible values (not recommended for hot-removal):
+
+- `online`: Places memory automatically between NORMAL and MOVABLE zone (may
+  prevent hot-remove)
+- `online_kernel`: Places memory in NORMAL zone (may prevent hot-remove)
+- `offline` (default): Memory requires manual onlining
+
+#### `memory_hotplug.memmap_on_memory` (optional)
+
+This parameter controls whether the kernel allocates memory map (`struct pages`)
+for hotplugged memory from the hotplugged memory itself, rather than from boot
+memory. Without this parameter, the kernel needs 64B for every 4KiB page in the
+boot memory. For example, it would need 262 MiB of free "boot" memory to hotplug
+16 GiB of memory. This parameter only works if the memory is not entirely
+hotplugged as MOVABLE.
+
+```
+memory_hotplug.memmap_on_memory=1 memhp_default_state=online
+```
+
+This configuration is recommended in case hot-removal is not a priority, and the
+hotpluggable memory area is very large.
+
+#### Additional Resources
+
+For more detailed and up-to-date information about memory hotplug in the Linux
+kernel, refer to the official kernel documentation:
+[Memory Hotplug](https://docs.kernel.org/admin-guide/mm/memory-hotplug.html)
+
+## Security Considerations
+
+**The `virtio-mem` device is a paravirtualized device requiring cooperation from
+a driver in the guest.**
+
+### Memory Protection
+
+Firecracker provides the following security guarantees:
+
+- **Memory that is never plugged is protected**: Memory that has never been
+  plugged before is protected from the guest by not making it available to the
+  guest via a KVM slot and by using `mprotect` to prevent access from device
+  emulation. Any attempt by the guest to access unplugged memory will result in
+  a fault and may crash the Firecracker process.
+- **Unplugged memory slots are protected**: Memory slots that have been
+  unplugged are removed from KVM and `mprotect`-ed. This requires the guest to
+  report contiguous blocks to be freed for the memory slot to be actually
+  protected.
+- **Unplugged memory blocks are freed**: When a memory block is unplugged, the
+  backing pages are freed, for example using `madvise(MADV_DONTNEED)` for anon
+  memory, returning memory to the host at block granularity.
+- **Memory isolation**: Unplugged memory never leaks between Firecracker
+  processes. Memory mappings use `MAP_PRIVATE` and `MAP_ANONYMOUS` flags,
+  ensuring that even if physical pages are reused, they are zeroed on access.
+
+### Trust Model
+
+While Firecracker enforces memory isolation at the host level, a compromised
+guest driver could:
+
+- Fail to plug or unplug memory as requested by the device
+- Attempt to access unplugged memory (will result in a fault and crash of
+  Firecracker)
+
+Users should:
+
+- Be prepared to handle cases where the guest doesn't cooperate with memory
+  operations by monitoring the GET API.
+- Implement host-level memory limits and monitoring, eg through cgroup.
+
+## Compatibility with Other Features
+
+`virtio-mem` is compatible with all Firecracker features. Below are some
+specific changes in the other features when using memory hotplugging.
+
+### Snapshots
+
+Full and diff snapshots will include the unplugged areas as sparse "holes" in
+the memory snapshot file. Sparse file support is recommended to efficiently
+handle the memory snapshot files.
+
+### Uffd
+
+The uffd handler will need to handle the entire hotpluggable memory range even
+if unplugged. The uffd handler may decide to unregister unplugged memory ranges
+(holes in the memory file). The uffd handler will also need to handle
+`UFFD_EVENT_REMOVE` events for hot-removed blocks, either unregistering the
+range or storing the information and returning an empty page on the next access.
+
+### Vhost-user
+
+`vhost-user` is fully supported, but Firecracker cannot guarantee protection of
+unplugged memory from a `vhost-user` backend. A malicious guest driver may be
+able to trick the backend to access unplugged memory. This is not possible in
+Firecracker itself as unplugged memory slots are `mprotect`-ed.