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Add a new section explaining the particularities of the microvm machine type for triggering a guest-initiated shut down. Signed-off-by: Sergio Lopez <slp@redhat.com> Message-Id: <20191115161338.42864-3-slp@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
130 lines
4.5 KiB
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130 lines
4.5 KiB
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====================
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microvm Machine Type
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====================
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``microvm`` is a machine type inspired by ``Firecracker`` and
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constructed after its machine model.
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It's a minimalist machine type without ``PCI`` nor ``ACPI`` support,
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designed for short-lived guests. microvm also establishes a baseline
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for benchmarking and optimizing both QEMU and guest operating systems,
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since it is optimized for both boot time and footprint.
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Supported devices
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-----------------
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The microvm machine type supports the following devices:
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- ISA bus
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- i8259 PIC (optional)
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- i8254 PIT (optional)
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- MC146818 RTC (optional)
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- One ISA serial port (optional)
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- LAPIC
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- IOAPIC (with kernel-irqchip=split by default)
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- kvmclock (if using KVM)
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- fw_cfg
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- Up to eight virtio-mmio devices (configured by the user)
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Limitations
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-----------
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Currently, microvm does *not* support the following features:
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- PCI-only devices.
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- Hotplug of any kind.
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- Live migration across QEMU versions.
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Using the microvm machine type
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------------------------------
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Machine-specific options
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~~~~~~~~~~~~~~~~~~~~~~~~
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It supports the following machine-specific options:
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- microvm.x-option-roms=bool (Set off to disable loading option ROMs)
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- microvm.pit=OnOffAuto (Enable i8254 PIT)
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- microvm.isa-serial=bool (Set off to disable the instantiation an ISA serial port)
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- microvm.pic=OnOffAuto (Enable i8259 PIC)
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- microvm.rtc=OnOffAuto (Enable MC146818 RTC)
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- microvm.auto-kernel-cmdline=bool (Set off to disable adding virtio-mmio devices to the kernel cmdline)
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Boot options
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~~~~~~~~~~~~
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By default, microvm uses ``qboot`` as its BIOS, to obtain better boot
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times, but it's also compatible with ``SeaBIOS``.
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As no current FW is able to boot from a block device using
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``virtio-mmio`` as its transport, a microvm-based VM needs to be run
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using a host-side kernel and, optionally, an initrd image.
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Running a microvm-based VM
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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By default, microvm aims for maximum compatibility, enabling both
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legacy and non-legacy devices. In this example, a VM is created
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without passing any additional machine-specific option, using the
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legacy ``ISA serial`` device as console::
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$ qemu-system-x86_64 -M microvm \
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-enable-kvm -cpu host -m 512m -smp 2 \
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-kernel vmlinux -append "earlyprintk=ttyS0 console=ttyS0 root=/dev/vda" \
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-nodefaults -no-user-config -nographic \
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-serial stdio \
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-drive id=test,file=test.img,format=raw,if=none \
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-device virtio-blk-device,drive=test \
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-netdev tap,id=tap0,script=no,downscript=no \
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-device virtio-net-device,netdev=tap0
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While the example above works, you might be interested in reducing the
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footprint further by disabling some legacy devices. If you're using
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``KVM``, you can disable the ``RTC``, making the Guest rely on
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``kvmclock`` exclusively. Additionally, if your host's CPUs have the
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``TSC_DEADLINE`` feature, you can also disable both the i8259 PIC and
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the i8254 PIT (make sure you're also emulating a CPU with such feature
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in the guest).
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This is an example of a VM with all optional legacy features
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disabled::
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$ qemu-system-x86_64 \
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-M microvm,x-option-roms=off,pit=off,pic=off,isa-serial=off,rtc=off \
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-enable-kvm -cpu host -m 512m -smp 2 \
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-kernel vmlinux -append "console=hvc0 root=/dev/vda" \
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-nodefaults -no-user-config -nographic \
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-chardev stdio,id=virtiocon0 \
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-device virtio-serial-device \
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-device virtconsole,chardev=virtiocon0 \
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-drive id=test,file=test.img,format=raw,if=none \
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-device virtio-blk-device,drive=test \
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-netdev tap,id=tap0,script=no,downscript=no \
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-device virtio-net-device,netdev=tap0
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Triggering a guest-initiated shut down
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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As the microvm machine type includes just a small set of system
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devices, some x86 mechanisms for rebooting or shutting down the
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system, like sending a key sequence to the keyboard or writing to an
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ACPI register, doesn't have any effect in the VM.
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The recommended way to trigger a guest-initiated shut down is by
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generating a ``triple-fault``, which will cause the VM to initiate a
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reboot. Additionally, if the ``-no-reboot`` argument is present in the
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command line, QEMU will detect this event and terminate its own
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execution gracefully.
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Linux does support this mechanism, but by default will only be used
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after other options have been tried and failed, causing the reboot to
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be delayed by a small number of seconds. It's possible to instruct it
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to try the triple-fault mechanism first, by adding ``reboot=t`` to the
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kernel's command line.
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