Types of virtualisation
The techniques for running one system inside another (emulation, full virtualisation, paravirtualisation, hardware assistance, OS level), what each one requires and what can be virtualised.
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Virtualisation: what is it?
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A technique that presents software with a simulated version of a resource (machine, storage, network) and lets several such versions share the same real resource.
A server, a desktop, an application, storage or a network can be virtualised: the principle is the same, a layer sits between the real resource and its users.
What does paravirtualisation require of the guest system?
- A modified kernel that calls the hypervisor instead of the hardware
- A processor with VT-x or AMD-V extensions, without which the guest does not boot
- The same kernel version as the host, which it uses directly
- An emulator that translates every guest instruction into host instructions
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A modified kernel that calls the hypervisor instead of the hardware
The guest kernel knows it is virtualised. Hardware assistance is not required (its historical advantage), the host kernel is only shared in OS-level virtualisation, and instruction-by-instruction translation is what emulation does.
On an Intel processor, hardware assistance is called …; on an AMD processor, it is called ….
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On an Intel processor, hardware assistance is called VT-x; on an AMD processor, it is called AMD-V.
On Linux, the
flagsline of/proc/cpuinfocontainsvmxfor Intel andsvmfor AMD when the extension is available.Emulation: what is it?
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A technique where software reproduces a processor instruction by instruction, including an architecture different from the host's.
It can run ARM code on an x86 PC, but every guest instruction costs several host instructions: it is the slowest technique.
VirtualBox returns
VT-x is disabled in the BIOS for all CPU modes (VERR_VMX_MSR_ALL_VMX_DISABLED)when starting a 64-bit VM. What should you do?- Reinstall the guest additions in the virtual machine's system
- Give the virtual machine more RAM in its settings
- Switch the virtual machine's network card to bridged mode
- Enable the processor's virtualisation extensions in the BIOS or UEFI
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Enable the processor's virtualisation extensions in the BIOS or UEFI
The message comes from the host: VT-x is turned off in the firmware, so the hypervisor has no reserved execution mode. Nothing set inside the VM (additions, memory, network) changes that.