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Christian Ehrhardt
Asked: 2021-04-27 21:42:50 +0800 CST

Qemu/KVM Postcopy migration fails in hirsute / v5.11 kernels (solved)

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postcopy migration isn't the default but can in some cases be very useful

With Ubuntu 21.04 Hirsute kernel being v5.11 postcopy migration will fail like:

virsh migrate --unsafe --live --postcopy --postcopy-after-precopy kvmguest-focal-postcopy qemu+ssh://10.85.93.248/system
error: internal error: unable to execute QEMU command 'migrate-set-capabilities': Postcopy is not supported
kernel

1 Answers

  1. The v5.11 kernel contains the following upstream change userfaultfd: add user-mode only option to unprivileged_userfaultfd sysctl knob

    This will also apply to e.g. a Focal-HWE kernel once on v5.11 or to Focal userspaces in a container under a Hirsute kernel (that is the example above).

    This was done for security reasons, if you want/need to re-enable un-limited userfault handling to be able to use postcopy again you'd want/need to set the control knob to one like:

    $ sudo sysctl -w "vm.unprivileged_userfaultfd=1"

    I mostly wanted users to be able to find the background and solution, therefore I also documented this in bug 1926249

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