KVM VirtIO paravirtualized drivers: why they matter

Written by Gionatan Danti on . Posted in Virtualization

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As you probably already know, there are basically two different schools in the virtualiztion champ:

  • the para-virtualization one, where a modified guest OS uses specific host-side syscall (hypercall) to do its “dirty work” with physical devices
  • the full hardware virtualization one (HVM), where the guest OS run unmodified and the host system “traps” when the guest try to access a physical device

The two approach are vastly different: the former requires extensive kernel modifications on both guest and host OSes but give you maximum performance, as both kernels are virtualization-aware and so they are optimized for the typical workload they experience. The latter approach is totally transparent to the guest OS and often do not require many kernel-level changes to the host side but, as the guest OS is not virtualization aware, it generally has lower performance.

So it appear that you had to do a conscious choice between performance and guest OS compatibility: the paravirtualized approach prioritize performance, while the HVM one prioritize compatibility. However, in this case it is possible to have the best of both worlds: by using para-virtualized guest device driver in an otherwise HVM environment, you can have compatibility and performance.

In short, a paravirtualizad device driver is a limited, targeted form of paravirtualization, useful when running specific guest OSes for which paravirtualization drivers are available. While being largely transparent to the guest OS (you simply need to install a driver), they relieve the virtualizer from emulating a real physical device (which is a complex operation, as it must emulate register, port, memory, ecc), substituting the emulation with some host-side syscall. The KVM-based framework to write paravirtualized drivers is called VirtIO.

Things are much more complex than this, of course. Anyway, in this article I am not going to explain in detail how a paravirtualized driver works, but to measure the performance implication of using it. Being a targeted paravirtualization form requiring guest-specific drivers, it is obvious that VirtIO is restricted to areas where it matter most, so disk and network subsystems are prime candidates for those paravirtualized drivers. Let see if, and how, both Linux (CentOS 6 x86-64) and Windows (Win2012R2 x64) are affected from that paravirtualized goodness.

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