KVM: Kernel-based Virtual Machine

Posted by Fraser Campbell Sun, 29 Oct 2006 19:19:00 GMT

10 days ago YAVM (yet another virtual machine) came to light.

Avi Kivity posted to linux-kernel presenting the “Kernel-based Virtual Machine”. The work is “a device driver and userspace component for Linux that utilizes hardware virtualization extensions such as Intel’s VT to create virtual machines running on a Linux host.”

This work sounds very interesting and is apparently already capable of running full virtual machines (running Linux and Windows is mentioned).

Here is Avi’s full introduction on linux-kernel:

The following patchset adds a driver for Intel’s hardware virtualization extensions to the x86 architecture. The driver adds a character device (/dev/kvm) that exposes the virtualization capabilities to userspace. Using this driver, a process can run a virtual machine (a “guest”) in a fully virtualized PC containing its own virtual hard disks, network adapters, and display.
Using this driver, one can start multiple virtual machines on a host. Each virtual machine is a process on the host; a virtual cpu is a thread in that process. kill(1), nice(1), top(1) work as expected.
In effect, the driver adds a third execution mode to the existing two: we now have kernel mode, user mode, and guest mode. Guest mode has its own address space mapping guest physical memory (which is accessible to user mode by mmap()ing /dev/kvm). Guest mode has no access to any I/O devices; any such access is intercepted and directed to user mode for emulation.
The driver supports i386 and x86_64 hosts and guests. All combinations are allowed except x86_64 guest on i386 host. For i386 guests and hosts, both pae and non-pae paging modes are supported.
SMP hosts and UP guests are supported. At the moment only Intel hardware is supported, but AMD virtualization support is being worked on.
Performance currently is non-stellar due to the naive implementation of the mmu virtualization, which throws away most of the shadow page table entries every context switch. We plan to address this in two ways:
- cache shadow page tables across tlv flushes
- wait until AMD and Intel release processors with nested page tables
Currently a virtual desktop is responsive but consumes a lot of CPU. Under Windows I tried playing pinball and watching a few flash movies; with a recent CPU one can hardly feel the virtualization. Linux/X is slower, probably due to X being in a separate process.
In addition to the driver, you need a slightly modified qemu to provide I/O device emulation and the BIOS.
Caveats:
- The Windows install currently bluescreens due to a problem with the virtual APIC. We are working on a fix. A temporary workaround is to use an existing image or install through qemu
- Windows 64-bit does not work. That’s also true for qemu, so it’s probably a problem with the device model.

Eventually a sourceforge project page is likely to appear at http://kvm.sourceforge.net/.

Comments

Leave a response

  1. frank said 6 days later:

    I can hardly wait to try this thing out. I’ve been planning to get a hand on a hardware VT enabled machine to play around with Xen’s HVM feature, but now there are more things to pay attention to.

    On an other note: I like this site, I wish to see it getting updates more often.

RSS feed for this post trackback uri

(leave url/email »)