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NEMU, a cloud hypervisor

NEMU is an open source hypervisor specifically built and designed to run modern cloud workloads on modern 64-bit Intel and ARM CPUs.

Rationale

Modern guest operating systems that host cloud workloads run on virtual hardware platforms that do not require any legacy hardware. Additonally modern CPUs used in data centers have advanced virtualization features that have eliminated the need for most CPU emulation.

There currently is no open source hypervisor solutions with a clear and narrow focus on running cloud specific workloads on modern CPUs. All available solutions have evolved over time and try to be fairly generic. They attempt to support a wide range of virtual hardware architectures and run on hardware that has varying degree of hardware virtualization support. This results in a need to provide a large set of legacy platforms and device models requiring CPU, device and platform emulation. As a consequence they are built on top of large and complex code bases.

NEMU on the other hand aims to leverage KVM, be narrow focused on exclusively running modern, cloud native workloads, on top of a limited set of hardware architectures and platforms. It assumes fairly recent CPUs and KVM allowing for the the elimination of most emulation logic.

This will allow for smaller code base, lower complexity and a reduced attack surface compared to existing solutions. It also gives more space for providing cloud specific optimizations and building a more performant hypervisor for the cloud. Reducing the size and complexity of the code allows for easier review, fuzz testing, modularization and future innovation.

QEMU base

QEMU is the current de facto standard open source cloud hypervisor. It has a rich set of features that have been developed and tested over time. This includes features such as live migration, PCI, Memory, NVDIMM and CPU hotplug, VFIO, mediated device passthrough and vhost-user. QEMU also has been the code base on which significant effort and innovation has been invested to create multiple performant I/O models

It also comes with a very large support for legacy features, for platforms and devices and is capable of running on a large number of hardware platforms. It also allows for cross platform emulation. One of its fundamental goal is about being as generic as possible and run on a large set of hardware and host a diversity of workloads. QEMU needed emulation support to be build into the code as hardware lacked critical virtualization features.

QEMU allows for build time configuration of some of its rich feature set. However there is quite a large amount of the code base that cannot be compiled out as the emulated platforms make assumptions about certain legacy devices being always present. QEMU also has abstractions within the code to support all of these legacy features.

NEMU

NEMU is based off QEMU and leverage its rich feature set, but with a much narrower focus. It leverages the performant, robust and stable QEMU codebase without the need to supporting the myriad of features, platforms and harware that are not relevant for the cloud.

The goal of NEMU is to retain the absolute minimal subset of the QEMU codebase that is required for the feature set described below. The QEMU code base will also be simplified to reduce the number of generic abstractions.

Requirements

NEMU provides a PCI virtio platform with support for vfio based device direct assigment and mediated device assigment support. It also aims to retain support for live migration, vhost-user and a build time configurable device hotplug support for PCI, memory, NVDIMM and CPU. NEMU will need to emulate a small subset of features including PCI host brige.

NEMU also introduces a new QEMU x86-64 machine type: virt. It is a purely virtual platform, that does not try to emulate any existing x86 chipset or legacy bus (ISA, SMBUS, etc) and offloads as many features to KVM as possible. This is a similar approach as the already existin AArch64 virt machine type and NEMU will only support the two virt machine types.

Below is a list of QEMU features that NEMU will retain and add.

High Level

  • KVM and KVM only based
  • Minimal emulation
  • Low latency
  • Low memory footprint
  • Low complexity
  • Small attack surface
  • 64-bit support only
  • Optional and build time configurable CPU, memory, PCI and NVDIMM hotplug
  • Machine to machine migration

Architectures

NEMU only supports two 64-bit CPU architectures:

  • x86-64
  • AArch64

Guest OS

  • 64-bit Linux

Guest Platforms

  • virt (x86-64) QEMU x86-64 virtual machine
  • virt (AArch64) QEMU AArch64 virtual machine

Host Platforms

  • Linux

Firmware and boot

  • UEFI
  • ACPI
    • Hardware Reduced ACPI
    • Optional hotplug support
      • CPU
      • Memory
      • NVDIMM
      • PCI devices
      • VFIO
      • vhost-user

Boot methods

  • UEFI boot

Memory

  • QEMU allocated memory
  • File mapped memory
  • Huge pages
  • Memory pinning

Devices

Models

  • virtio
    • blk
    • console
    • crypto
    • pci-net
    • rng-pci
    • scsi
      • virtio
      • vhost
    • 9pfs
    • vhost-user-scsi
    • vhost-user-net
    • vhost-user-blk
    • vhost-vsock-pci
  • vfio
    • network
    • mediated device
    • storage
    • rdma
  • NVDIMM
  • TPM
    • vTPM
    • Host TPM passthrough
  • SCSI controller
  • PCI controller (pci-lite)

Block

  • cdrom
  • nvme
  • ceph/rbd

Guest Image Formats

  • QCOW2
  • RAW
  • VHD

Migration

  • Network based over TLS
  • File based (Local migration)

Monitoring

  • QMP
  • QAPI

To be discussed

  • 64-bit Windows Server (headless)
  • qboot
  • Graphic Console
  • virtio-block-crypto
  • QEMU client support as modules
    • iscsi
    • nbd
    • nfs
    • gluster
  • RDMA live migration
  • SLIRP
  • Guest agent