NVMM
Created:08-09-2018
Updated:07-10-2018

NetBSD Virtual Machine Monitor


The NVMM driver provides hardware-accelerated virtualization support on NetBSD. It is made of an ~MI frontend, to which MD backends can be plugged. A virtualization API is provided in libnvmm, that allows to easily create and manage virtual machines via NVMM. Two additional components are shipped as demonstrators, toyvirt and smallkern: the former is a toy virtualizer, that executes in a VM the 64bit ELF binary given as argument, the latter is an example of such binary.

Download


The source code of NVMM, plus the associated tools, can be downloaded here.

Technical details


NVMM can support up to 128 virtual machines, each having a maximum of 256 VCPUs and 4GB of RAM.

Each virtual machine is granted access to most of the CPU registers: the GPRs (obviously), the Segment Registers, the Control Registers, the Debug Registers, the FPU (x87 and SSE), and several MSRs.

Events can be injected in the virtual machines, to emulate device interrupts. A delay mechanism is used, and allows VMM software to schedule the interrupt right when the VCPU can receive it. NMIs can be injected as well, and use a similar mechanism.

The host must always be x86_64, but the guest has no constraint on the mode, so it can be x86_32, PAE, real mode, and so on.

The TSC of each VCPU is always re-based on the host CPU it is executing on, and is therefore guaranteed to increase regardless of the host CPU. However, it may not increase monotonically, because it is not possible to fully hide the host effects on the guest during #VMEXITs.

When there are more VCPUs than the host TLB can deal with, NVMM uses a shared ASID, and flushes the shared-ASID VCPUs on each VM switch.

The different intercepts are configured in such a way that they cover everything that needs to be emulated. In particular, the LAPIC can be emulated by VMM software, by intercepting reads/writes to the LAPIC page in memory, and monitoring changes to CR8 in the exit state.

Support


One backend is supported currently, x86-SVM, for x86 AMD CPUs.