From aa60d34e13b88a9d4d608e7b4ed45ff9a6ed32eb Mon Sep 17 00:00:00 2001
From: "prompt.ac/@jeffrey" <mail@aesthetic.computer>
Date: Tue, 14 Apr 2026 11:07:58 -0700
Subject: [PATCH] oven: persistent /kernel-build volume + skip no-op .config
 rewrites
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Previously every native build copied the whole linux-$KVER tree from
/cache into an ephemeral $BUILD dir, so ccache helped per-file but every
link, modpost, kallsyms, and bzImage assembly pass ran fresh. Now the
kernel tree lives on a named Docker volume (ac-os-kbuild) mounted at
/kernel-build; docker-build.sh seeds it from /cache on first run and
keeps the .o files between oven runs. A .kver sentinel wipes the tree
when the kernel version bumps.

Also: cmp .config against the repo copy before overwriting — the blind
`cp` was touching .config's mtime every run, invalidating kbuild's own
dependency graph even when the config hadn't changed.

Expected win: code-only changes drop from ~10 min kernel rebuild to
~1-2 min (only changed .c files recompile + bzImage re-links with new
initramfs). Config changes still do the full rebuild.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 fedac/native/docker-build.sh | 41 ++++++++++++++++++++++++++++++------
 oven/native-builder.mjs      |  2 +-
 2 files changed, 36 insertions(+), 7 deletions(-)

diff --git a/fedac/native/docker-build.sh b/fedac/native/docker-build.sh
index 81458c68c6..dcf9b911db 100644
--- a/fedac/native/docker-build.sh
+++ b/fedac/native/docker-build.sh
@@ -627,22 +627,51 @@ log "  Initramfs: $((INITRAMFS_SIZE / 1048576))MB compressed"
 # Step 4: Build kernel with embedded initramfs
 # ══════════════════════════════════════════════
 log "Step 4/4: Building kernel..."
-LINUX_DIR="$BUILD/linux-$KVER"
+
+# Persistent kernel build tree lives on a Docker volume mounted at /kernel-build.
+# This preserves object files between oven runs so ccache + kbuild's own
+# dependency tracker produce true incremental rebuilds. Fall back to ephemeral
+# $BUILD when the volume isn't present (e.g. running docker-build.sh locally
+# without the oven wrapper).
+if [ -d /kernel-build ] && [ -w /kernel-build ]; then
+    KBUILD_ROOT="/kernel-build"
+    log "  Using persistent kernel build tree at $KBUILD_ROOT"
+else
+    KBUILD_ROOT="$BUILD"
+    log "  No persistent volume — kernel tree at $KBUILD_ROOT (ephemeral)"
+fi
+LINUX_DIR="$KBUILD_ROOT/linux-$KVER"
+
+# Version sentinel: if the persisted tree is for a different kernel version,
+# wipe it before reinitializing from the Docker image's cached source.
+SENTINEL="$KBUILD_ROOT/.kver"
+if [ -f "$SENTINEL" ] && [ "$(cat "$SENTINEL")" != "$KVER" ]; then
+    log "  Kernel version changed ($(cat "$SENTINEL") → $KVER), wiping build tree"
+    rm -rf "$KBUILD_ROOT/linux-"*
+    rm -f "$SENTINEL"
+fi
 
 # Use cached kernel source or download
 if [ ! -f "$LINUX_DIR/Makefile" ]; then
     if [ -d "/cache/linux-$KVER" ]; then
-        log "  Using cached Linux $KVER..."
-        cp -a "/cache/linux-$KVER" "$BUILD/"
+        log "  Initializing $KBUILD_ROOT from cached Linux $KVER..."
+        cp -a "/cache/linux-$KVER" "$KBUILD_ROOT/"
     else
         log "  Downloading Linux $KVER..."
-        cd "$BUILD"
+        cd "$KBUILD_ROOT"
         curl -sL "https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-${KVER}.tar.xz" | tar xJ
     fi
+    echo "$KVER" > "$SENTINEL"
 fi
 
-# Copy config
-cp "$NATIVE/kernel/config-minimal" "$LINUX_DIR/.config"
+# Copy config only if it differs — overwriting touches mtime and invalidates
+# large swaths of kbuild's dependency graph, forcing full rebuilds.
+if ! cmp -s "$NATIVE/kernel/config-minimal" "$LINUX_DIR/.config"; then
+    log "  Config changed, updating .config"
+    cp "$NATIVE/kernel/config-minimal" "$LINUX_DIR/.config"
+else
+    log "  Config unchanged, preserving existing build tree"
+fi
 
 # Stage built-in firmware blobs referenced by CONFIG_EXTRA_FIRMWARE
 # into a container-local directory and rewrite CONFIG_EXTRA_FIRMWARE_DIR.
diff --git a/oven/native-builder.mjs b/oven/native-builder.mjs
index f77d735f59..84b0924ba4 100644
--- a/oven/native-builder.mjs
+++ b/oven/native-builder.mjs
@@ -471,7 +471,7 @@ async function runBuildJob(job) {
       addLogLine(job, "stdout", "Phase 2: Compiling C kernel in Docker...");
       const cidFile = `/tmp/oven-cid-${job.id}`;
       await runPhase(job, "build", "bash", ["-c", [
-        `CID=$(docker create -e AC_BUILD_NAME=${buildName} -e AC_GIT_HASH=${buildGitHash} -e AC_BUILD_TS=${buildTs} -v ac-os-ccache:/ccache ac-os-builder)`,
+        `CID=$(docker create -e AC_BUILD_NAME=${buildName} -e AC_GIT_HASH=${buildGitHash} -e AC_BUILD_TS=${buildTs} -v ac-os-ccache:/ccache -v ac-os-kbuild:/kernel-build ac-os-builder)`,
         `echo $CID > ${cidFile}`,
         `docker start -a $CID`,
       ].join(" && ")], repoDir);
-- 
2.51.2