Bug 700369

Summary: qemu-system-x86_64 says 'KVM not supported for this target'
Product: [Fedora] Fedora Reporter: Marek Goldmann <mgoldman>
Component: qemuAssignee: Justin M. Forbes <jforbes>
Status: CLOSED ERRATA QA Contact: Fedora Extras Quality Assurance <extras-qa>
Severity: unspecified Docs Contact:
Priority: unspecified    
Version: 15CC: amit.shah, armbru, berrange, dwmw2, ehabkost, gcosta, itamar, jaswinder, jforbes, knoel, markmc, mbooth, ondrejj, rjones, scottt.tw, virt-maint
Target Milestone: ---   
Target Release: ---   
Hardware: Unspecified   
OS: Linux   
Fixed In Version: libguestfs-1.10.4-1.fc15 Doc Type: Bug Fix
Doc Text:
Story Points: ---
Clone Of: Environment:
Last Closed: 2011-05-17 15:09:02 EDT Type: ---
Regression: --- Mount Type: ---
Documentation: --- CRM:
Verified Versions: Category: ---
oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: ---

Description Marek Goldmann 2011-04-28 04:56:39 EDT
Description of problem:

http://libguestfs.org/guestfs.3.html#guestfs_config doesn't help either because it's overridden by -enable-kvm switch later. We should check if we're running on bare metal and enable KVN only in this case, in other cases use -no-kvm switch.

See below guestfish session:

$ LIBGUESTFS_QEMU=/usr/bin/qemu-system-x86_64 guestfish -v -a build/appliances/x86_64/fedora/14/f14-jeos/fedora-plugin/tmp/f14-jeos-sda.raw

Welcome to guestfish, the libguestfs filesystem interactive shell for
editing virtual machine filesystems.

Type: 'help' for help on commands
      'man' to read the manual
      'quit' to quit the shell

><fs> launch
libguestfs: [00000ms] febootstrap-supermin-helper --verbose -f checksum '/usr/lib64/guestfs/supermin.d' x86_64
supermin helper [00000ms] whitelist = (not specified), host_cpu = x86_64, kernel = (null), initrd = (null), appliance = (null)
supermin helper [00000ms] inputs[0] = /usr/lib64/guestfs/supermin.d
checking modpath /lib/modules/ is a directory
picked vmlinuz- because modpath /lib/modules/ exists
supermin helper [00000ms] finished creating kernel
supermin helper [00000ms] visiting /usr/lib64/guestfs/supermin.d
supermin helper [00000ms] visiting /usr/lib64/guestfs/supermin.d/base.img
supermin helper [00001ms] visiting /usr/lib64/guestfs/supermin.d/daemon.img
supermin helper [00001ms] visiting /usr/lib64/guestfs/supermin.d/hostfiles
supermin helper [00038ms] visiting /usr/lib64/guestfs/supermin.d/init.img
supermin helper [00170ms] finished creating appliance
libguestfs: [00178ms] begin testing qemu features
libguestfs: [00220ms] finished testing qemu features
libguestfs: accept_from_daemon: 0x20b25b0 g->state = 1
libguestfs: [00232ms] /usr/bin/qemu-system-x86_64 \
    -drive file=build/appliances/x86_64/fedora/14/f14-jeos/fedora-plugin/tmp/f14-jeos-sda.raw,cache=off,if=virtio \
    -nodefconfig \
    -enable-kvm \
    -nodefaults \
    -nographic \
    -m 500 \
    -no-reboot \
    -no-hpet \
    -device virtio-serial \
    -serial stdio \
    -chardev socket,path=/tmp/libguestfsdxVSvH/guestfsd.sock,id=channel0 \
    -device virtserialport,chardev=channel0,name=org.libguestfs.channel.0 \
    -kernel /var/tmp/.guestfs-0/kernel.14705 \
    -initrd /var/tmp/.guestfs-0/initrd.14705 \
    -append 'panic=1 console=ttyS0 udevtimeout=300 noapic acpi=off printk.time=1 cgroup_disable=memory selinux=0 guestfs_verbose=1 TERM=xterm-color ' \
    -drive file=/var/tmp/.guestfs-0/root.14705,snapshot=on,if=virtio,cache=unsafe
KVM not supported for this target
libguestfs: error: unexpected end of file when reading from daemon
libguestfs: child_cleanup: 0x20b25b0: child process died

Version-Release number of selected component (if applicable):


How reproducible:

Always on non-bare metal hosts.

$ guestfish -v -a /dev/null launch
Actual results:


Expected results:

No error, surprisingly :)

Additional info:

Fedora 15
Comment 1 Richard W.M. Jones 2011-04-28 05:01:27 EDT
We already check if KVM is available by checking if /dev/kvm

    if (qemu_supports (g, "-enable-kvm") &&
        is_openable (g, "/dev/kvm", O_RDWR))
      add_cmdline (g, "-enable-kvm");

Can you see if /dev/kvm exists, and what permissions it has?

Does this machine use udev, or is /dev directory hard-coded?
Comment 2 Richard W.M. Jones 2011-04-28 05:28:33 EDT
The problem is that qemu-system-x86_64 advertizes the
-enable-kvm option:

$ qemu-system-x86_64 --help | grep enable-kvm
-enable-kvm     enable KVM full virtualization support

But then completely rejects it:

$ qemu-system-x86_64 -enable-kvm -hdc /dev/null
KVM not supported for this target

(It doesn't even open /dev/kvm or check for it in any

This is a qemu bug basically.
Comment 3 Richard W.M. Jones 2011-04-28 05:36:52 EDT
Markus, can you suggest the correct / reliable way that
we should be using so that we get KVM if it's available,
but otherwise fall back to TCG, which will work across all
versions of qemu and KVM?
Comment 4 Markus Armbruster 2011-04-28 07:09:26 EDT
Unfortunately, there is no way to ask for "KVM or else TCG" that works across versions.

With qemu-kvm, it is the default.

With upstream qemu, it's simply not possible.

You could simply assume qemu-kvm semantics on the hypothesis that anyone who cares for KVM will use qemu-kvm for production.  If you use qemu, you get TCG.

If you must have KVM even with upstream qemu, you can try with -enable-kvm, recognize "no KVM support" failure, retry without.  Recognizing the failure will be relatively brittle, I'm afraid.  Or start an extra instance just to run "info kvm" in the monitor.

Please don't shoot the messenger...

Additional info:

upstream qemu   |  default  |-enable-kvm
KVM available   | disabled  |  enabled
KVM unavailable | disabled  |    fail

qemu-kvm old    |  default  |-enable-kvm|  -no-kvm
KVM available   |  enabled* |  enabled  |  disabled
KVM unavailable | disabled  | disabled* |  disabled

qemu-kvm new    |  default  |-enable-kvm|  -no-kvm
KVM available   |  enabled* |  enabled  |  disabled
KVM unavailable | disabled  |    fail+  |  disabled
Comment 5 Richard W.M. Jones 2011-05-12 07:11:15 EDT
OK, but you see why this is why we want proper qemu
Comment 6 Markus Armbruster 2011-05-12 08:36:06 EDT
I understand why a robust way to test for feature availability is desirable.  Unfortunately, upstream has rejected all attempts to provide it so far.  Another case of perfect being the enemy of good.
Comment 7 Richard W.M. Jones 2011-05-12 12:59:13 EDT
Re: Marek suggested testing for vmx|svm in /proc/cpuinfo flags.

That's not going to work.  The problem is not that the hardware
doesn't support hardware virt, but that qemu advertizes the -enable-kvm
option but doesn't implement it.  It's entirely a problem with qemu.
Comment 8 Markus Armbruster 2011-05-12 14:31:26 EDT
Want me to try to get upstream qemu's default changed to match qemu-kvm's?  Chance of success is unpredictable.
Comment 9 Richard W.M. Jones 2011-05-12 14:40:18 EDT
A better thing (perhaps easier) would be to remove
-enable-kvm from the qemu help output when it's not
actually supported by the built binary ...
Comment 10 Richard W.M. Jones 2011-05-17 09:00:33 EDT
Patch posted upstream:


This requires the new qemu -machine option which is not yet
widely available, but it seems to be the best we can do.
Comment 11 Richard W.M. Jones 2011-05-17 15:09:02 EDT
Pushed upstream:


and available in libguestfs >= 1.11.7.
Comment 12 Fedora Update System 2011-06-13 16:52:35 EDT
libguestfs-1.10.4-1.fc15 has been submitted as an update for Fedora 15.
Comment 13 Fedora Update System 2011-06-23 23:50:15 EDT
libguestfs-1.10.4-1.fc15 has been pushed to the Fedora 15 stable repository.  If problems still persist, please make note of it in this bug report.