Add a doc for Microdroid kernel building and prebuilt updating
Bug: N/A
Test: N/A
Change-Id: Icc4d70ffc47482854259c3c20de62f41805eb734
diff --git a/microdroid/kernel/README.md b/microdroid/kernel/README.md
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+# Microdroid kernel
+
+This directory contains prebuilt images of the Linux kernel that is used in
+Microdroid. The kernel is built from the same source tree as Generic Kernel
+Image (GKI), but with a different config where most of the config items are
+turned off to make the kernel fast & slim.
+
+## How to build the Microdroid kernels
+
+### Checkout the GKI source code.
+
+```bash
+repo init -u https://android.googlesource.com/kernel/manifest -b android14-5.15
+repo sync
+```
+
+### Build the Microdroid kernels manually
+
+For ARM64
+```bash
+FAST_BUILD=1 BUILD_CONFIG=common-modules/virtual-device/build.config.microdroid.aarch64 build/build.sh
+```
+
+For x86\_64,
+```bash
+FAST_BUILD=1 BUILD_CONFIG=common-modules/virtual-device/build.config.microdroid.x86_64 build/build.sh
+```
+
+Note that `FAST_BUILD=1` is not mandatory, but will make your build much faster.
+
+## How to update Microdroid kernel prebuilts
+
+### For manually built kernels (only for your own development)
+
+Copy the built kernel image to the Android source tree directly, and build the virt APEX.
+
+For ARM64,
+```bash
+cp out/android14-5.15/dist/Image <android_checkout>/packages/modules/Virtualization/microdroid/kernel/arm64/kernel-5.15
+```
+
+For x86\_64,
+```bash
+cp out/android14-5.15/dist/bzImage <android_checkout>/packages/modules/Virtualization/microdroid/kernel/arm64/kernel-5.15
+```
+
+### For official updates
+
+Use the `download_from_ci` script to automatically fetch the built images from
+a specific `<build_id>` and make commits with nice history in the message.
+
+```bash
+cd <android_checkout>/packages/modules/Virtualization
+repo start <topic_name>
+cd <kernel_checkout>
+ANDROID_BUILD_TOP=<android_checkout> ./build/kernel/gki/download_from_ci --update-microdroid -b <bug_id> <build_id>
+cd <android_checkout>/packages/modules/Virtualization
+repo upload .
+```