What is the usefulness of docker/setup-qemu-action ?

[bplessis-swi]

Hi,

Following some diagnose attempt on our multi-platform build i kept seeing two different things, one was the documented recommendation of setting up qemu (and consequently binfmt) prior to buildx and the other one was that the builds weren't actually using this version of qemu

Having some runners working under kubernetes i thought it was a kube/permission issue but even runner on "bare-metal" os actually use some version of qemu backed inside the buildkit image:

/usr/bin/containerd-shim-runc-v2 -namespace moby -id 2b66215448d40bb6b6e5bc9d8dc7472d9682231a2c21c455d2d9c2f599a8e825 -address /run/containerd/
\_ /sbin/docker-init -- buildkitd --allow-insecure-entitlement security.insecure --allow-insecure-entitlement network.host
\_ buildkitd --allow-insecure-entitlement security.insecure --allow-insecure-entitlement network.host
\_ buildkit-runc --log /var/lib/buildkit/runc-overlayfs/executor/runc-log.json --log-format json run --bundle /var/lib/buildkit/runc-o
\_ /dev/.buildkit_qemu_emulator /bin/sh -c apt-get -y update &&     apt-get -y upgrade &&     rm -rf /var/lib/apt/lists/ &&     rm
\_ .buildkit_qemu_emulator -0 apt-get /usr/bin/apt-get -y update

And the moby/buildkit image documentation actually say

When your build needs to run a binary for architecture that is not supported natively by your host, it gets executed using a QEMU user-mode emulator. You do not need to set up QEMU manually in most cases.