Qemu-system-x86_64 Does Not Support Fw_cfg Option

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I am trying to run CoreOS (Container Linux) as a guest OS using KVM/QEMU. One of their suggestions is to pass config info into the VM
using the “fw_cfg” option . When I try this I get the error “-fw_cfg invalid option”. If I run the vm with the help option it does not list the fw_cfg option. However, according to the qemu documentation, see here
, they support this option.

The command used to run the vm is:

/usr/bin/qemu-system-x86_64 -name CL-IGN-1 -m 1024 \
-net nic,vlan=0,model=virtio \
-net user,vlan=0,hostfwd=tcp::2222-:22,hostname=CL-IGN-1 \
-drive if=virtio,file=./coreos_production_qemu_image.img \
-fw_cfg name=opt/com.coreos/config,file=./CL-IGN-1.ignconfig

qemu-system-x86_64: -fw_cfg: invalid option

I also tried using /usr/libexec/qemu-kvm but got the same error.

I am running::

CentOS Linux release 7.3.1611 (Core)



2 thoughts on - Qemu-system-x86_64 Does Not Support Fw_cfg Option

  • Hello, I think (based also on your qemu-kvm version man page), that the feature is not present in qemu-kvm version provided by base CentOS. But you can use packages provided by the Virtualization SIG
    Good idea especially if virtualization is core service for your server. See here the announcement:

    So you can follow what detailed there:
    yum install CentOS-release-qemu-ev yum install qemu-kvm-ev

    I reproduced a situation on a CentOS 7.3 server where qemu-kvm is already present and you can do

    yum install CentOS-release-qemu-ev

    and then

    yum update qemu-kvm-ev
    … Dependencies Resolved

    =================================================================================================== Package Arch Version Repository Size
    qemu-img-ev x86_64 10:2.6.0-28.el7.10.1
    CentOS-qemu-ev 1.0 M
    replacing qemu-img.x86_64 10:1.5.3-126.el7_3.10
    qemu-kvm-common-ev x86_64 10:2.6.0-28.el7.10.1
    CentOS-qemu-ev 516 k
    replacing qemu-kvm-common.x86_64 10:1.5.3-126.el7_3.10
    qemu-kvm-ev x86_64 10:2.6.0-28.el7.10.1
    CentOS-qemu-ev 2.5 M
    replacing qemu-kvm.x86_64 10:1.5.3-126.el7_3.10

    Transaction Summary
    ===================================================================================================Install 3 Packages

    so it seems it automatically obsoletes the “standard” packages, because they overlap in filesand you can have only one installed

    And now you should have the -fw_cfg option.

    BTW: the version of the hypervisor used by Virtualization SIG is the same used by the Virtualization Platform project named oVirt (
    http://www.ovirt.org), upstream for commercial RHEV solution, so rock solid, I’m currently using it extensively ;-)

    HIH, Gianluca

  • Many thanks for your quick response Gianluca. I installed the “ev”
    packages and now I can pass the “fw_cfg” option to the vm.