illumos-kvm: KVM for illumos

KVM is the kernel virtual machine, a framework for the in-kernel acceleration of QEMU. illumos-kvm is a port of KVM to illumos, taking advantage of illumos-specific constructs like DTrace, cyclics, mdb, kstat, OS virtualization, network virtualization, ZFS, etc. It is derived from the KVM source for Linux 2.6.34, the longterm source for which may be found here:

git://git.kernel.org/pub/scm/linux/kernel/git/longterm/linux-2.6.34.y.git

To date, this implementation has been verified with a wide range of guest operating systems including illumos itself (both SmartOS and OpenIndiana distributions), FreeBSD, Plan 9, QNX, ChromeOS, HaikuOS, Microsoft Windows and Linux.

The design center for this work is to use the virtualization features made available in the microprocessor -- and in particular, Intel's VMX. As such, behavior on microprocessors that do not support VMX -- and more specifically, the extended page tables (EPT) found in second generation VMX support -- should be graceful failure, not degraded operation.

Divergences from KVM

Divergences from KVM fall into several broad categories: some functionality has been removed or not implemented because it is obviated by features of illumos (e.g., the custom tracing facility built into KVM); some functionality has been removed because it is only relevant to hardware that lacks virtualization support (e.g., older x86 hardware) or on hardware for which illumos lacks support (e.g., PPC, s390); and some functionality has been removed because the implementation complexity was simply too great relative to its value.

Of this latter category, three areas of divergence merit special note. First, there is no support for pageable guest memory (that is, guest memory is locked down). While this is an opinionated decision at some level (in our experience, memory oversell leads to unacceptable pathologies in all but the idlest of workloads), we would welcome the work to integrate the KVM MMU notifier support into illumos-kvm.

Second (and relatedly), illumos itself has no support for kernel same-page mapping (KSM) as found in Linux. While illumos could in principle add such support, it is our experience that the memory that accrues from this is not sufficiently significant to pay for the increase in implementation and operator complexity.

Finally, there is no support currently for AMD SVM. This is not a value judgement of AMD's technology, but rather a reflection of limited engineering and testing resources. (In the spirit of full disclosure, it should be said that the sponsor of illumos-kvm, Joyent, is an Intel-funded company -- but the lack of AMD support reflects only engineering prioritization and lack of testing infrastructure; AMD SVM support would be most welcome should someone in the community be so motivated as to port and test it.)

Building illumos-kvm

Preparation

Edit the Makefile and appropriately set the path for the KERNEL_SOURCE directory to point to the root of a checked out and built illumos directory. Building illumos KVM requires several recent additions to illumos, so be sure your illumos is up to date.

Verify that you have gcc 4.4.4 installed that is used to build illumos. If you are not building this with SmartOS, you may need to modify the Makefile such that CC is pointing to the correct gcc.

Verify that you either have SUNWmake or GNU make installed.

Building

To build, simply use the default make target:

$ make

To check style, header files, and other various nits:

$ make check

Installing illumos-kvm

System requirements

To run illumos-kvm, you will need an illumos that has the fix for issue 1347 (integrated on 2011-08-11). Further, your machine will need to support VMX. To see if your machine supports VMX, run isainfo -v and look for vmx, e.g.:

  % isainfo -v
  64-bit amd64 applications
        vmx sse4.2 sse4.1 ssse3 popcnt tscp cx16 sse3 sse2 sse fxsr mmx 
        cmov amd_sysc cx8 tsc fpu 
  32-bit i386 applications
        vmx sse4.2 sse4.1 ssse3 popcnt tscp ahf cx16 sse3 sse2 sse fxsr mmx 
        cmov sep cx8 tsc fpu 

If you do not see vmx in this output, the kvm driver will be unable to attach.

Required binaries

There are two mandatory artifacts to install, and two optional component:

• kvm is the driver itself
• kvm.conf is the driver configuration file
• kvm.so is the mdb module
• JOY_kvm_link.so is the devfsadm plugin

On the target machine, place kvm in /kernel/drv/amd64 and kvm.conf in /kernel/drv. Place JOY_kvm_link.so in /usr/lib/devfsadm/linkmod then:

# add_drv kvm

You can verify that the driver installed and attached properly by checking for its presence in /dev.

# ls -l /dev/kvm

Running illumos-kvm

To run KVM, you will need the build product of the illumos-kvm-cmd repo: qemu-system-x86_64; please follow the instructions in the illumos-kvm-cmd repo to execute QEMU such that KVM is enabled.

Monitoring illumos-kvm

Once one or more VMs are running, there is a variety of tooling to help understand the operating characteristics of the system.

kvmstat

The kvmstat command, found in the illumos repository, can be used to monitor VMs. For example, here is one second of kvmstat output from a machine running two VMs (one 2 VCPU instance running Linux; another 4 VCPU instance running the illumos-derived SmartOS):

   pid vcpu |  exits :  haltx   irqx  irqwx    iox  mmiox |   irqs   emul   eptv
  4668    0 |     23 :      6      0      0      1      0 |      6     16      0
  4668    1 |     25 :      6      1      0      1      0 |      6     16      0
  5026    0 |  17833 :    223   2946    707    106      0 |   3379  13315      0
  5026    1 |  18687 :    244   2761    512      0      0 |   3085  14803      0
  5026    2 |  15696 :    194   3452    542      0      0 |   3568  11230      0
  5026    3 |  16822 :    244   2817    487      0      0 |   3100  12963      0

As for the meaning of the columns, they are explained with kvmstat -h:

  # kvmstat -h
  Usage: kvmstat [interval [count]]

    Displays statistics for running kernel virtual machines, with one line
    per virtual CPU.  All statistics are reported as per-second rates.

    The columns are as follows:

      pid    =>  identifier of process controlling the virtual CPU
      vcpu   =>  virtual CPU identifier relative to its virtual machine
      exits  =>  virtual machine exits for the virtual CPU
      haltx  =>  virtual machine exits due to the HLT instruction
      irqx   =>  virtual machine exits due to a pending external interrupt
      irqwx  =>  virtual machine exits due to an open interrupt window
      iox    =>  virtual machine exits due to an I/O instruction
      mmiox  =>  virtual machine exits due to memory mapped I/O 
      irqs   =>  interrupts injected into the virtual CPU
      emul   =>  instructions emulated in the kernel
      eptv   =>  extended page table violations

kstat

As one might expect, kvmstat is implemented in terms of kstat. You can use kstat(1) to browse the kstats from the kvm module:

   # kstat -m kvm
   ...
   module: kvm                      instance: 0     
   name:   vcpu-4                   class:    misc
    crtime                          4407.142410068
    exits                           5367443
    fpu-reload                      57302
    halt-exits                      317275
    halt-wakeup                     8991
    host-state-reload               503920
    hypercalls                      0
    insn-emulation                  3043881
    inst-emulation-fail             0
    invlpg                          0
    io-exits                        237191
    irq-exits                       1668
    irq-injections                  320339
    irq-window-exits                1635
    mmio-exits                      617
    nmi-injections                  0
    nmi-window-exits                0
    pf-fixed                        163629
    pf-guest                        0
    pid                             3949
    request-irq-exits               0
    signal-exits                    460
    snaptime                        43219.723435123
    zonename                        global
   
   module: kvm                      instance: 4     
   name:   vm                       class:    misc
    crtime                          4407.1241134
    lpages                          0
    mmu-cache-miss                  950
    mmu-flooded                     0
    mmu-pte-updated                 0
    mmu-pte-write                   56360
    mmu-pte-zapped                  0
    mmu-recycled                    0
    mmu-unsync-page                 0
    pid                             3949
    remote-tlb-flush                1511
    snaptime                        43219.723875091
    zonename                        global

DTrace

While there is not currently a stable KVM provider, there are many SDT probes in KVM; dtrace -l -m sdt:kvm to list these.

Of these, of particular note are the kvm-guest-entry and kvm-guest-exit probes, which fire upon entry to and exit from a guest virtual machine. To determine context, one can use the vmregs variable present in illumos.

For example, here's a simple script that shows histograms of time spent in VM guests on a per-PID and per-VCPU basis:

    #pragma D option quiet
    
    kvm-guest-entry
    {
            self->entry = timestamp;
    }
    
    kvm-guest-exit
    /self->entry/
    {
            @[pid, vmregs[VMX_VIRTUAL_PROCESSOR_ID]] =
                quantize(timestamp - self->entry);
    }
    
    END
    {
            printa("pid %d, vcpu %d: %@d\n", @);
    }

Here's what the output of running the above might look like:

  pid 3949, vcpu 1: 
       value  ------------- Distribution ------------- count    
         512 |                                         0        
        1024 |@@@@@@@@@@@@@                            26805    
        2048 |@@@@@                                    11641    
        4096 |@@@@@@@                                  14187    
        8192 |@                                        1559     
       16384 |@                                        2931     
       32768 |@@@                                      5653     
       65536 |@@@@                                     8385     
      131072 |@@@                                      6926     
      262144 |@@@                                      6639     
      524288 |                                         785      
     1048576 |                                         0        

There are many other ways in which DTrace can be used to understand either host or guest behavior; see the tools subdirectory from some sample D scripts.

mdb

The kvm.so build product is an mdb module that contains several useful commands, including a kvm walker to iterate over all struct kvm structures.

Contributing to illumos-kvm

Unless and until its volume dictate that it be elsewhere, illumos KVM discussion should be on the illumos-developer mailing list. Contributions are happily accepted; please send patches to illumos-developer.