diff --git a/.github/workflows/cmake.yml b/.github/workflows/cmake.yml
index 25a3b812..4c21521a 100644
--- a/.github/workflows/cmake.yml
+++ b/.github/workflows/cmake.yml
@@ -9,6 +9,7 @@ jobs:
       matrix:
         include:
           - name: Ubuntu GCC
+            enabled: true
             os: ubuntu-latest
             compiler: gcc
             cflags: -Wall -Wextra
@@ -16,6 +17,7 @@ jobs:
             cmake-args: -DMINIZIP_ENABLE_BZIP2=ON
 
           - name: Ubuntu GCC -O3
+            enabled: true
             os: ubuntu-latest
             compiler: gcc
             cflags: -O3 -Wall -Wextra
@@ -23,6 +25,7 @@ jobs:
             cmake-args: -DMINIZIP_ENABLE_BZIP2=ON
 
           - name: Ubuntu Clang
+            enabled: true
             os: ubuntu-latest
             compiler: clang
             cflags: -Wall -Wextra
@@ -30,6 +33,7 @@ jobs:
             cmake-args: -DMINIZIP_ENABLE_BZIP2=ON
 
           - name: Ubuntu Clang Debug
+            enabled: true
             os: ubuntu-latest
             compiler: clang
             cflags: -Wall -Wextra
@@ -38,6 +42,7 @@ jobs:
             cmake-args: -DMINIZIP_ENABLE_BZIP2=ON
 
           - name: Windows MSVC Win32
+            enabled: true
             os: windows-latest
             compiler: cl
             cflags: /W3
@@ -45,6 +50,7 @@ jobs:
             pkgtgt: PACKAGE
 
           - name: Windows MSVC Win64
+            enabled: true
             os: windows-latest
             compiler: cl
             cflags: /W3
@@ -52,6 +58,7 @@ jobs:
             pkgtgt: PACKAGE
 
           - name: Windows GCC
+            enabled: true
             os: windows-latest
             compiler: gcc
             cflags: -Wall -Wextra
@@ -59,6 +66,7 @@ jobs:
             pkgtgt: package
 
           - name: macOS Clang
+            enabled: true
             os: macos-latest
             compiler: clang
             cflags: -Wall -Wextra
@@ -66,44 +74,59 @@ jobs:
             cmake-args: -DMINIZIP_ENABLE_BZIP2=ON
 
           - name: macOS GCC
+            enabled: true
             os: macos-latest
             compiler: gcc-12
             cflags: -Wall -Wextra
             pkgtgt: package
             cmake-args: -DMINIZIP_ENABLE_BZIP2=ON
 
+          - name: S390X Native GCC
+            enabled: ${{ github.repository == 'fneddy/zlib' && 'true' || false }}
+            os: ${{ github.repository == 'fneddy/zlib' && 'S390X' || 'ubuntu-latest' }}
+            compiler: gcc
+            cflags: -Wall -Wextra
+            cmake-args: -DMINIZIP_ENABLE_BZIP2=OFF
+            pkgtgt: package
+
     steps:
     - name: Checkout repository
+      if: matrix.enabled
       uses: actions/checkout@v4
 
     - name: Install packages (Windows)
-      if: runner.os == 'Windows'
+      if: runner.os == 'Windows' && matrix.enabled
       run: |
         choco install --no-progress ninja
 
     - name: Install packages (Linux)
-      if: runner.os == 'Linux'
+      if: runner.os == 'Linux' &&  matrix.os != 'S390X' && matrix.enabled
       run: |
         sudo apt install libbz2-dev
+
     - name: Generate project files
+      if: matrix.enabled
       run: cmake -S . -B ../build ${{ matrix.cmake-args }} -D CMAKE_BUILD_TYPE=${{ matrix.build-config || 'Release' }} -DZLIB_BUILD_MINIZIP=ON
       env:
         CC: ${{ matrix.compiler }}
         CFLAGS: ${{ matrix.cflags }}
 
     - name: Compile source code
+      if: matrix.enabled
       run: cmake --build ../build --config ${{ matrix.build-config || 'Release' }}
 
     - name: Run test cases
+      if: matrix.enabled
       run: ctest -C Release --output-on-failure --max-width 120
       working-directory: ../build
 
     - name: create packages
+      if: matrix.enabled
       run: cmake --build ../build --config ${{ matrix.build-config || 'Release' }} -t ${{ matrix.pkgtgt }}
 
     - name: Upload build errors
       uses: actions/upload-artifact@v4
-      if: failure()
+      if: failure() && matrix.enabled == true
       with:
         name: ${{ matrix.name }} (cmake)
         path: |
diff --git a/.github/workflows/configure.yml b/.github/workflows/configure.yml
index f19272c7..e9e4df6b 100644
--- a/.github/workflows/configure.yml
+++ b/.github/workflows/configure.yml
@@ -9,12 +9,14 @@ jobs:
       matrix:
         include:
           - name: Ubuntu GCC
+            enabled: true
             os: ubuntu-latest
             compiler: gcc
             configure-args: --warn
 
           # Test out of source builds
           - name: Ubuntu GCC OSB
+            enabled: true
             os: ubuntu-latest
             compiler: gcc
             configure-args: --warn
@@ -22,6 +24,7 @@ jobs:
             src-dir: ../zlib
 
           - name: Ubuntu GCC ARM SF
+            enabled: true
             os: ubuntu-latest
             compiler: arm-linux-gnueabi-gcc
             configure-args: --warn
@@ -30,6 +33,7 @@ jobs:
             qemu-run: qemu-arm -L /usr/arm-linux-gnueabi
 
           - name: Ubuntu GCC ARM HF
+            enabled: true
             os: ubuntu-latest
             compiler: arm-linux-gnueabihf-gcc
             configure-args: --warn
@@ -38,6 +42,7 @@ jobs:
             qemu-run: qemu-arm -L /usr/arm-linux-gnueabihf
 
           - name: Ubuntu GCC AARCH64
+            enabled: true
             os: ubuntu-latest
             compiler: aarch64-linux-gnu-gcc
             configure-args: --warn
@@ -46,6 +51,7 @@ jobs:
             qemu-run: qemu-aarch64 -L /usr/aarch64-linux-gnu
 
           - name: Ubuntu GCC PPC
+            enabled: true
             os: ubuntu-latest
             compiler: powerpc-linux-gnu-gcc
             configure-args: --warn --static
@@ -56,6 +62,7 @@ jobs:
             ldflags: -static
 
           - name: Ubuntu GCC PPC64
+            enabled: true
             os: ubuntu-latest
             compiler: powerpc64-linux-gnu-gcc
             configure-args: --warn --static
@@ -66,6 +73,7 @@ jobs:
             ldflags: -static
 
           - name: Ubuntu GCC PPC64LE
+            enabled: true
             os: ubuntu-latest
             compiler: powerpc64le-linux-gnu-gcc
             configure-args: --warn
@@ -73,7 +81,9 @@ jobs:
             packages: qemu-system qemu-user gcc-powerpc64le-linux-gnu libc-dev-ppc64el-cross
             qemu-run: qemu-ppc64le -L /usr/powerpc64le-linux-gnu
 
+          # if on base repo use the native workflow runner, else QEMU
           - name: Ubuntu GCC S390X
+            enabled: true
             os: ubuntu-latest
             compiler: s390x-linux-gnu-gcc
             configure-args: --warn --static
@@ -83,27 +93,37 @@ jobs:
             cflags: -static
             ldflags: -static
 
+          - name: Native S390X GCC
+            enabled: ${{ github.repository == 'fneddy/zlib' && 'true' || false }}
+            os: ${{ github.repository == 'fneddy/zlib' && 'S390X' || 'ubuntu-latest' }}
+            compiler: gcc
+            configure-args: --warn
+
           - name: macOS GCC
+            enabled: true
             os: macos-latest
             compiler: gcc-12
             configure-args: --warn
 
           - name: macOS Clang
+            enabled: true
             os: macos-latest
             compiler: clang
             configure-args: --warn
 
     steps:
     - name: Checkout repository
+      if: matrix.enabled
       uses: actions/checkout@v4
 
     - name: Install packages (Ubuntu)
-      if: runner.os == 'Linux' && matrix.packages
+      if: runner.os == 'Linux' && matrix.packages && matrix.enabled
       run: |
         sudo apt-get update
         sudo apt-get install -y ${{ matrix.packages }}
 
     - name: Generate project files
+      if: matrix.enabled
       run: |
         [ -d ${{ matrix.build-dir || '.' }} ] || mkdir ${{ matrix.build-dir || '.' }}
         cd ${{ matrix.build-dir || '.' }}
@@ -115,10 +135,12 @@ jobs:
         CHOST: ${{ matrix.chost }}
 
     - name: Compile source code
+      if: matrix.enabled
       run: make -j2
       working-directory: ${{ matrix.build-dir }}
 
     - name: Run test cases
+      if: matrix.enabled
       run: |
         make test
         make cover
@@ -128,7 +150,7 @@ jobs:
 
     - name: Upload build errors
       uses: actions/upload-artifact@v4
-      if: failure()
+      if: failure() && matrix.enabled
       with:
         name: ${{ matrix.name }} (configure)
         path: |
diff --git a/contrib/README.contrib b/contrib/README.contrib
index 5e5f9505..36f14ae9 100644
--- a/contrib/README.contrib
+++ b/contrib/README.contrib
@@ -55,3 +55,6 @@ untgz/      by Pedro A. Aranda Gutierrez <paag@tid.es>
 vstudio/    by Gilles Vollant <info@winimage.com>
         Building a minizip-enhanced zlib with Microsoft Visual Studio
         Includes vc11 from kreuzerkrieg and vc12 from davispuh
+
+s390x/      by Eduard Stefes <eduard.stefes@ibm.com
+        Scripts to create S390X github action workers.
\ No newline at end of file
diff --git a/contrib/s390x/README b/contrib/s390x/README
new file mode 100644
index 00000000..f8847e8c
--- /dev/null
+++ b/contrib/s390x/README
@@ -0,0 +1,64 @@
+
+# Testing
+
+> [!NOTE]  
+> This is a copy of the s390x self-hosted action runner scripts from
+> https://github.com/zlib-ng/zlib-ng/tree/a0fa24710c8faa1a746a20cfd5c7c24571e15ca4/arch/s390/self-hosted-builder
+
+Given complexity of DFLTCC machine instruction, it is not clear whether
+QEMU TCG will ever support it. At the time of writing, one has to have
+access to an IBM z15+ VM or LPAR in order to test DFLTCC support. Since
+DFLTCC is a non-privileged instruction, neither special VM/LPAR
+configuration nor root are required.
+
+zlib CI uses a self-hosted builder, provided by marist university 
+for the DFLTCC testing. There is no official IBM Z GitHub Actions runner, 
+so we build one inspired by `anup-kodlekere/gaplib`.
+Future updates to actions-runner might need an updated patch. The .net
+version number patch has been separated into a separate file to avoid a
+need for constantly changing the patch.
+
+## Configuring the builder.
+
+### Install prerequisites.
+```
+sudo dnf install podman
+```
+
+### Create a config file, needs github personal access token.
+Access token needs permissions; Repo Admin RW, Org Self-hosted runners RW.
+For details, consult
+https://docs.github.com/en/rest/actions/self-hosted-runners?apiVersion=2022-11-28#create-a-registration-token-for-a-repository
+
+#### Create file /etc/actions-runner:
+```
+REPO=<owner>/<name>
+PAT_TOKEN=<github_pat_***>
+```
+
+#### Set permissions on /etc/actions-runner:
+```
+chmod 0600 /etc/actions-runner
+```
+
+### Add actions-runner service.
+```
+sudo cp self-hosted-builder/actions-runner.service /etc/systemd/system/
+sudo systemctl daemon-reload
+```
+
+### Autostart actions-runner.
+```
+$ sudo systemctl enable --now actions-runner
+```
+
+### Add auto-rebuild cronjob
+```
+sudo cp self-hosted-builder/actions-runner-rebuild.sh /etc/cron.weekly/
+chmod +x /etc/cron.weekly/actions-runner-rebuild.sh
+```
+
+## Building / Rebuilding the container
+```
+sudo /etc/cron.weekly/actions-runner-rebuild.sh
+```
diff --git a/contrib/s390x/self-hosted-builder/actions-runner b/contrib/s390x/self-hosted-builder/actions-runner
new file mode 100755
index 00000000..9d443f93
--- /dev/null
+++ b/contrib/s390x/self-hosted-builder/actions-runner
@@ -0,0 +1,59 @@
+#!/bin/bash
+
+#
+# Ephemeral runner startup script.
+#
+# Expects the following environment variables:
+#
+# - REPO=<owner>
+# - PAT_TOKEN=<github_pat_***>
+#
+
+set -e -u
+
+# Validate required environment variables
+if [ -z "${REPO:-}" ] || [ -z "${PAT_TOKEN:-}" ]; then
+    echo "Error: REPO and/or PAT_TOKEN environment variables not found"
+    exit 1
+fi
+
+# Check the cached registration token.
+TOKEN_FILE=registration-token.json
+if [ -f $TOKEN_FILE ]; then
+    set +e
+    EXPIRES=$(jq --raw-output .EXPIRES "$TOKEN_FILE" 2>/dev/null)
+    STATUS=$?
+    set -e
+else
+    STATUS=1
+fi
+if [[ $STATUS -ne 0 || $(date +%s) -ge $(date -d "$EXPIRES" +%s) ]]; then
+    # Refresh the cached registration token.
+    curl \
+        -X POST \
+        -H "Accept: application/vnd.github+json" \
+        -H "Authorization: Bearer $PAT_TOKEN" \
+        "https://api.github.com/repos/$REPO/actions/runners/registration-token" \
+        -o "$TOKEN_FILE"
+fi
+
+REG_TOKEN=$(jq --raw-output .token "$TOKEN_FILE")
+if [ $REG_TOKEN = "null" ]; then
+    echo "Failed to get registration token"
+    exit 1
+fi
+
+# (Re-)register the runner.
+./config.sh remove --token "$REG_TOKEN" || true
+set -x
+./config.sh \
+    --url "https://github.com/$REPO" \
+    --token "$REG_TOKEN" \
+    --unattended \
+    --disableupdate \
+    --replace \
+    --labels S390X \
+    --ephemeral
+
+# Run one job.
+./run.sh
diff --git a/contrib/s390x/self-hosted-builder/actions-runner-rebuild.sh b/contrib/s390x/self-hosted-builder/actions-runner-rebuild.sh
new file mode 100644
index 00000000..a3b9bcd2
--- /dev/null
+++ b/contrib/s390x/self-hosted-builder/actions-runner-rebuild.sh
@@ -0,0 +1,49 @@
+#!/usr/bin/bash
+set -ex
+
+TMPDIR="$(mktemp -d)"
+
+if [ -f actions-runner.Dockerfile ]; then
+    MODE=1
+    cp actions-runner.Dockerfile actions-runner entrypoint $TMPDIR
+    cd $TMPDIR
+else
+    MODE=2
+    cd $TMPDIR
+    wget https://raw.githubusercontent.com/fneddy/zlib/refs/heads/s390x_native_ci/contrib/s390x/self-hosted-builder/actions-runner.Dockerfile
+    wget https://raw.githubusercontent.com/fneddy/zlib/refs/heads/s390x_native_ci/contrib/s390x/self-hosted-builder/actions-runner
+    wget https://raw.githubusercontent.com/fneddy/zlib/refs/heads/s390x_native_ci/contrib/s390x/self-hosted-builder/entrypoint
+fi
+
+# Stop service
+systemctl stop actions-runner || true
+
+# Delete old container
+podman container rm gaplib-actions-runner || true
+
+# Delete old image
+podman image rm localhost/zlib/actions-runner || true
+
+# Build new image
+podman build --squash -f actions-runner.Dockerfile --tag zlib/actions-runner . 2>&1 | tee /var/log/actions-runner-build.log
+
+# Create new container
+podman create --replace --name=gaplib-actions-runner --env-file=/etc/actions-runner --init \
+       --volume=actions-runner-temp:/home/actions-runner zlib/actions-runner 2>&1 | tee -a /var/log/actions-runner-build.log
+
+# Start service
+systemctl start actions-runner || true
+
+# Cleanup
+podman image prune -af || true
+
+# Clean up tempfile
+if [ "$MODE" == "2" ] ; then
+    cd $TMPDIR
+    rm actions-runner.Dockerfile
+    rm actions-runner
+    rm entrypoint
+    cd ..
+    rmdir $TMPDIR
+    echo "Deleted tempfiles."
+fi
diff --git a/contrib/s390x/self-hosted-builder/actions-runner.Dockerfile b/contrib/s390x/self-hosted-builder/actions-runner.Dockerfile
new file mode 100644
index 00000000..d8213d00
--- /dev/null
+++ b/contrib/s390x/self-hosted-builder/actions-runner.Dockerfile
@@ -0,0 +1,45 @@
+# Self-Hosted IBM Z Github Actions Runner.
+
+FROM    almalinux:9
+
+RUN     dnf update -y -q && \
+        dnf install -y -q --enablerepo=crb wget git which sudo jq sed \
+            cmake make automake autoconf m4 libtool ninja-build python3-pip \
+            gcc gcc-c++ clang llvm-toolset glibc-all-langpacks langpacks-en \
+            glibc-static libstdc++-static libstdc++-devel libxslt-devel libxml2-devel
+
+RUN     dnf install -y -q dotnet-sdk-8.0 && \
+        echo "Using SDK - `dotnet --version`"
+
+RUN     cd /tmp && \
+        git clone -q https://github.com/actions/runner && \
+        cd runner && \
+        git checkout main -b build && \
+        wget https://github.com/anup-kodlekere/gaplib/raw/refs/heads/main/build-files/runner-sdk-8.s390x.patch && \
+        git apply runner-sdk-8.s390x.patch && \
+        sed -i'' -e /version/s/8......\"$/$8.0.100\"/ src/global.json
+
+
+RUN     cd /tmp/runner/src && \
+        ./dev.sh layout && \
+        ./dev.sh package && \
+        rm -rf /root/.dotnet /root/.nuget
+
+RUN     useradd -c "Action Runner" -m actions-runner && \
+        usermod -L actions-runner
+
+RUN     tar -xf /tmp/runner/_package/*.tar.gz -C /home/actions-runner && \
+        chown -R actions-runner:actions-runner /home/actions-runner
+
+# Cleanup
+RUN     rm -rf /tmp/runner /var/cache/dnf/* /tmp/runner.patch /tmp/global.json && \
+        dnf clean all
+
+USER    actions-runner
+
+# Scripts.
+COPY    --chmod=555 entrypoint /usr/bin/
+COPY    --chmod=555 actions-runner /usr/bin/
+WORKDIR /home/actions-runner
+ENTRYPOINT ["/usr/bin/entrypoint"]
+CMD     ["/usr/bin/actions-runner"]
diff --git a/contrib/s390x/self-hosted-builder/actions-runner.service b/contrib/s390x/self-hosted-builder/actions-runner.service
new file mode 100644
index 00000000..79560cde
--- /dev/null
+++ b/contrib/s390x/self-hosted-builder/actions-runner.service
@@ -0,0 +1,18 @@
+[Unit]
+Description=Podman container: Gaplib Github Actions Runner
+Wants=network-online.target
+After=network-online.target
+StartLimitIntervalSec=1
+RequiresMountsFor=/run/user/1001/containers
+
+[Service]
+Environment=PODMAN_SYSTEMD_UNIT=%n
+Restart=always
+TimeoutStopSec=61
+ExecStart=/usr/bin/podman start gaplib-actions-runner
+ExecStop=/usr/bin/podman stop -t 30 gaplib-actions-runner
+ExecStopPost=/usr/bin/podman stop -t 10 gaplib-actions-runner
+Type=forking
+
+[Install]
+WantedBy=default.target
diff --git a/contrib/s390x/self-hosted-builder/entrypoint b/contrib/s390x/self-hosted-builder/entrypoint
new file mode 100755
index 00000000..eb8772be
--- /dev/null
+++ b/contrib/s390x/self-hosted-builder/entrypoint
@@ -0,0 +1,30 @@
+#!/bin/bash
+
+#
+# Container entrypoint that waits for all spawned processes.
+#
+
+set -e -u
+
+# Create a FIFO and start reading from its read end.
+tempdir=$(mktemp -d "/tmp/done.XXXXXXXXXX")
+trap 'rm -r "$tempdir"' EXIT
+done="$tempdir/pipe"
+mkfifo "$done"
+cat "$done" & waiter=$!
+
+# Start the workload. Its descendants will inherit the FIFO's write end.
+status=0
+if [ "$#" -eq 0 ]; then
+    bash 9>"$done" || status=$?
+else
+    "$@" 9>"$done" || status=$?
+fi
+
+# When the workload and all of its descendants exit, the FIFO's write end will
+# be closed and `cat "$done"` will exit. Wait until it happens. This is needed
+# in order to handle SelfUpdater, which the workload may start in background
+# before exiting.
+wait "$waiter"
+
+exit "$status"