Emulating a big-endian s390x with QEMU#

multiarch/qemu-user-static#

This is the first command to run. It does something magical to your Docker installation:

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docker run --rm --privileged multiarch/qemu-user-static:register --reset

The qemu-user-static README says:

multiarch/qemu-user-static and multiarch/qemu-user-static:register images execute the register script that registers below kind of /proc/sys/fs/binfmt_misc/qemu-$arch files for all supported processors except the current one in it when running the container.

It continues:

The --reset option is implemented at the register script that executes find /proc/sys/fs/binfmt_misc -type f -name 'qemu-*' -exec sh -c 'echo -1 > {}' \; to remove binfmt_misc entry files before register the entry.

I don’t understand what this means. But running this command was essential for the next command to work.

multiarch/ubuntu-core#

Having run that command, the following command drops you into a shell in an emulated s390x machine running Ubuntu Focal:

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docker run -it multiarch/ubuntu-core:s390x-focal /bin/bash

Using -focal gives you Python 3.8. I previously tried s390x-bionic but that gave me Python 3.6.

You don’t actually get Python until you install it, like so:

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apt-get -y update && apt-get -y install python3

This will take a while. I think it’s slower because the hardware is being emulated.

Now you can check the Python version and confirm that the byte order is big-endian like this:

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root@ea63e288ce49:/# python3 --version
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Python 3.8.10
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root@ea63e288ce49:/# python3 -c 'import sys; print(sys.byteorder)'
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big

Doing this in GitHub Actions#

I figured out the following recipe for running this in GitHub Actions.

In this example I’m cloning my sqlite-fts4 repo and running the tests in it as well:

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name: QEMU to run s390x-focal
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on:
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  push:
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  workflow_dispatch:
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jobs:
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  one:
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    runs-on: ubuntu-latest
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    steps:
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    - name: Setup multiarch/qemu-user-static
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      run: |
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        docker run --rm --privileged multiarch/qemu-user-static:register --reset
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    - name: ubuntu-core:s390x-focal
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      uses: docker://multiarch/ubuntu-core:s390x-focal
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      with:
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        args: >
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          bash -c
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          "uname -a &&
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          lscpu | grep Endian &&
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          apt-get -y update &&
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          apt-get -y install python3 git python3.8-venv &&
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          python3 --version &&
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          python3 -c 'import sys; print(sys.byteorder)' &&
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          git clone https://github.com/simonw/sqlite-fts4 &&
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          cd sqlite-fts4 &&
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          python3 -m venv venv &&
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          source venv/bin/activate &&
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          pip install -e '.[test]' &&
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          pytest
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          "