neonvm-kernel: Fix upgrade instruction (#1172)

Fix the instruction for kernel upgrades, add separate commands for arm64
and amd64 config upgrades.

Rerun the commands with current version. It has produced some diff, most
likely due to ubuntu upgrade in
#1159.

Split off from #1170

neonvm-kernel/Dockerfile.kernel-builder

@@ -64,7 +64,7 @@ FROM --platform=linux/amd64 alpine:3.19 AS kernel_amd64
 ARG KERNEL_VERSION
 COPY --from=build_amd64 /build/linux-${KERNEL_VERSION}/arch/x86/boot/bzImage /vmlinuz
 
-# Build the kernel on amd64
+# Build the kernel on arm64
 FROM build-deps AS build_arm64
 ARG KERNEL_VERSION
 ADD linux-config-aarch64-${KERNEL_VERSION} linux-${KERNEL_VERSION}/.config

neonvm-kernel/README.md

@@ -18,26 +18,52 @@ in use, although this can be overridden on an individual VM basis using the
 Assuming a plain upgrade (i.e. no additional features to enable), upgrading the kernel can be done
 with the following sequence of actions:
 
+### On amd64 (x64)
+
+1. On the host, run:
+   ```sh
+   cd neonvm-kernel # this directory
+   docker build --build-arg KERNEL_VERSION=$NEW_VERSION --target build-deps -t kernel-build-deps -f Dockerfile.kernel-builder .
+   docker run --rm -v $PWD:/host --name kernel-build -it kernel-build-deps bash
+   ```
+2. Then, inside the container, run:
+   ```sh
+   cd linux-$NEW_VERSION
+   cp /host/linux-config-amd64-6.1.92 .config # Copy current config in
+   make menuconfig ARCH=x86_64
+   # do nothing; just save and exit, overwriting .config
+   cp .config /host/linux-config-amd64-$NEW_VERSION # NOTE: Different from existing!
+   ```
+3. Back on the host, finish with:
+   ```sh
+   # compare the two versions
+   diff linux-config-amd64-6.1.92 linux-config-amd64-$NEW_VERSION
+   # If all looks good, delete the old version. This is required so auto-selection works.
+   rm linux-config-amd64-6.1.92
+   ```
+
+### On arm64 (aarch64 ARM)
+
 1. On the host, run:
    ```sh
    cd neonvm-kernel # this directory
-   docker build --build-arg KERNEL_VERSION=$NEW_VERSION --platform linux/x86_64 --target build-deps -t kernel-build-deps -f Dockerfile.kernel-builder .
+   docker build --build-arg KERNEL_VERSION=$NEW_VERSION --target build-deps -t kernel-build-deps -f Dockerfile.kernel-builder .
    docker run --rm -v $PWD:/host --name kernel-build -it kernel-build-deps bash
    ```
 2. Then, inside the container, run:
    ```sh
    cd linux-$NEW_VERSION
-   cp /host/linux-config-6.1.92 .config # Copy current config in
-   make menuconfig
+   cp /host/linux-config-aarch64-6.1.92 .config # Copy current config in
+   make menuconfig ARCH=arm64
    # do nothing; just save and exit, overwriting .config
-   cp .config /host/linux-config-$NEW_VERSION # NOTE: Different from existing!
+   cp .config /host/linux-config-aarch64-$NEW_VERSION # NOTE: Different from existing!
    ```
 3. Back on the host, finish with:
    ```sh
    # compare the two versions
-   diff linux-config-6.1.92 linux-config-$NEW_VERSION
+   diff linux-config-aarch64-6.1.92 linux-config-aarch64-$NEW_VERSION
    # If all looks good, delete the old version. This is required so auto-selection works.
-   rm linux-config-6.1.92
+   rm linux-config-aarch64-6.1.92
    ```
 
 Afterwards, it's probably also good to do a search-and-replace repo-wide to update all places that