microvm.rst (4580B)
1'microvm' virtual platform (``microvm``) 2======================================== 3 4``microvm`` is a machine type inspired by ``Firecracker`` and 5constructed after its machine model. 6 7It's a minimalist machine type without ``PCI`` nor ``ACPI`` support, 8designed for short-lived guests. microvm also establishes a baseline 9for benchmarking and optimizing both QEMU and guest operating systems, 10since it is optimized for both boot time and footprint. 11 12 13Supported devices 14----------------- 15 16The microvm machine type supports the following devices: 17 18- ISA bus 19- i8259 PIC (optional) 20- i8254 PIT (optional) 21- MC146818 RTC (optional) 22- One ISA serial port (optional) 23- LAPIC 24- IOAPIC (with kernel-irqchip=split by default) 25- kvmclock (if using KVM) 26- fw_cfg 27- Up to eight virtio-mmio devices (configured by the user) 28 29 30Limitations 31----------- 32 33Currently, microvm does *not* support the following features: 34 35- PCI-only devices. 36- Hotplug of any kind. 37- Live migration across QEMU versions. 38 39 40Using the microvm machine type 41------------------------------ 42 43Machine-specific options 44~~~~~~~~~~~~~~~~~~~~~~~~ 45 46It supports the following machine-specific options: 47 48- microvm.x-option-roms=bool (Set off to disable loading option ROMs) 49- microvm.pit=OnOffAuto (Enable i8254 PIT) 50- microvm.isa-serial=bool (Set off to disable the instantiation an ISA serial port) 51- microvm.pic=OnOffAuto (Enable i8259 PIC) 52- microvm.rtc=OnOffAuto (Enable MC146818 RTC) 53- microvm.auto-kernel-cmdline=bool (Set off to disable adding virtio-mmio devices to the kernel cmdline) 54 55 56Boot options 57~~~~~~~~~~~~ 58 59By default, microvm uses ``qboot`` as its BIOS, to obtain better boot 60times, but it's also compatible with ``SeaBIOS``. 61 62As no current FW is able to boot from a block device using 63``virtio-mmio`` as its transport, a microvm-based VM needs to be run 64using a host-side kernel and, optionally, an initrd image. 65 66 67Running a microvm-based VM 68~~~~~~~~~~~~~~~~~~~~~~~~~~ 69 70By default, microvm aims for maximum compatibility, enabling both 71legacy and non-legacy devices. In this example, a VM is created 72without passing any additional machine-specific option, using the 73legacy ``ISA serial`` device as console:: 74 75 $ qemu-system-x86_64 -M microvm \ 76 -enable-kvm -cpu host -m 512m -smp 2 \ 77 -kernel vmlinux -append "earlyprintk=ttyS0 console=ttyS0 root=/dev/vda" \ 78 -nodefaults -no-user-config -nographic \ 79 -serial stdio \ 80 -drive id=test,file=test.img,format=raw,if=none \ 81 -device virtio-blk-device,drive=test \ 82 -netdev tap,id=tap0,script=no,downscript=no \ 83 -device virtio-net-device,netdev=tap0 84 85While the example above works, you might be interested in reducing the 86footprint further by disabling some legacy devices. If you're using 87``KVM``, you can disable the ``RTC``, making the Guest rely on 88``kvmclock`` exclusively. Additionally, if your host's CPUs have the 89``TSC_DEADLINE`` feature, you can also disable both the i8259 PIC and 90the i8254 PIT (make sure you're also emulating a CPU with such feature 91in the guest). 92 93This is an example of a VM with all optional legacy features 94disabled:: 95 96 $ qemu-system-x86_64 \ 97 -M microvm,x-option-roms=off,pit=off,pic=off,isa-serial=off,rtc=off \ 98 -enable-kvm -cpu host -m 512m -smp 2 \ 99 -kernel vmlinux -append "console=hvc0 root=/dev/vda" \ 100 -nodefaults -no-user-config -nographic \ 101 -chardev stdio,id=virtiocon0 \ 102 -device virtio-serial-device \ 103 -device virtconsole,chardev=virtiocon0 \ 104 -drive id=test,file=test.img,format=raw,if=none \ 105 -device virtio-blk-device,drive=test \ 106 -netdev tap,id=tap0,script=no,downscript=no \ 107 -device virtio-net-device,netdev=tap0 108 109 110Triggering a guest-initiated shut down 111~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 112 113As the microvm machine type includes just a small set of system 114devices, some x86 mechanisms for rebooting or shutting down the 115system, like sending a key sequence to the keyboard or writing to an 116ACPI register, doesn't have any effect in the VM. 117 118The recommended way to trigger a guest-initiated shut down is by 119generating a ``triple-fault``, which will cause the VM to initiate a 120reboot. Additionally, if the ``-no-reboot`` argument is present in the 121command line, QEMU will detect this event and terminate its own 122execution gracefully. 123 124Linux does support this mechanism, but by default will only be used 125after other options have been tried and failed, causing the reboot to 126be delayed by a small number of seconds. It's possible to instruct it 127to try the triple-fault mechanism first, by adding ``reboot=t`` to the 128kernel's command line.