feat: parallelize build per (kernel_version, arch)

Generate the build matrix dynamically from kernel_versions.txt so each
(version, arch) pair runs on its own runner. With N versions × 2 archs
that's 2N parallel jobs (e.g. 2 versions -> 4 runners).

build.sh already supports single-version mode, so no script changes;
the matrix job emits a JSON include list that the build job fans out
over.

Author: ValentaTomas

.github/workflows/release.yml

@@ -9,28 +9,44 @@ permissions:
   id-token: write
 
 jobs:
+  matrix:
+    runs-on: ubuntu-24.04
+    outputs:
+      build_matrix: ${{ steps.gen.outputs.matrix }}
+    steps:
+      - uses: actions/checkout@v4
+      - id: gen
+        run: |
+          python3 - >> "$GITHUB_OUTPUT" <<'PY'
+          import json
+          versions = []
+          for line in open("kernel_versions.txt").read().splitlines():
+              v = line.split("#", 1)[0].strip()
+              if v:
+                  versions.append(v)
+          include = []
+          for v in versions:
+              include.append({"version": v, "arch": "amd64", "target_arch": "x86_64", "runner": "ubuntu-24.04"})
+              include.append({"version": v, "arch": "arm64", "target_arch": "arm64", "runner": "ubuntu-24.04-arm"})
+          print(f"matrix={json.dumps({'include': include})}")
+          PY
+
   build:
-    name: Build kernels (${{ matrix.arch }})
+    needs: matrix
+    name: Build ${{ matrix.version }} (${{ matrix.arch }})
     strategy:
       fail-fast: false
-      matrix:
-        include:
-          - arch: amd64
-            target_arch: x86_64
-            runner: ubuntu-24.04
-          - arch: arm64
-            target_arch: arm64
-            runner: ubuntu-24.04-arm
+      matrix: ${{ fromJson(needs.matrix.outputs.build_matrix) }}
     runs-on: ${{ matrix.runner }}
     steps:
       - uses: actions/checkout@v4
 
-      - name: Build kernels
-        run: sudo TARGET_ARCH=${{ matrix.target_arch }} ./build.sh
+      - name: Build kernel
+        run: sudo ./build.sh "${{ matrix.version }}" "${{ matrix.target_arch }}"
 
       - uses: actions/upload-artifact@v4
         with:
-          name: kernels-${{ matrix.arch }}
+          name: kernel-${{ matrix.version }}-${{ matrix.arch }}
           path: ./builds
           retention-days: 7
 

README.md

@@ -29,7 +29,7 @@ This project builds custom Linux kernels for Firecracker microVMs from the same
 
 The **Build & Release** workflow runs in two modes:
 
-- **On every pull request**: builds every kernel in `kernel_versions.txt` for `amd64` and `arm64` in parallel and uploads the binaries as workflow artifacts (downloadable from the PR's checks tab) so reviewers can inspect them. No release or GCS upload happens.
+- **On every pull request**: builds every (kernel version × arch) combination from `kernel_versions.txt` in parallel (one runner per pair) and uploads the binaries as workflow artifacts (downloadable from the PR's checks tab) so reviewers can inspect them. No release or GCS upload happens.
 - **Manually (workflow_dispatch)**: pick the branch in the GitHub UI and run. It does the same build as a PR and additionally creates a GitHub release tagged `YYYY.MM.DD` (with a `.N` suffix for additional runs the same day) containing every binary, and uploads them to GCS.
 
 Release asset naming for that commit: