volt-vmm/scripts/build-kernel.sh

#!/usr/bin/env bash
#
# build-kernel.sh - Build an optimized microVM kernel for Volt
#
# This script downloads and builds a minimal Linux kernel configured
# specifically for fast-booting microVMs with KVM virtualization.
#
# Requirements:
#   - gcc, make, flex, bison, libelf-dev, libssl-dev
#   - ~2GB disk space, ~10 min build time
#
# Output: kernels/vmlinux (uncompressed kernel for direct boot)

set -euo pipefail

SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
PROJECT_DIR="$(dirname "$SCRIPT_DIR")"
BUILD_DIR="${PROJECT_DIR}/.build/kernel"
OUTPUT_DIR="${PROJECT_DIR}/kernels"

# Kernel version - LTS for stability
KERNEL_VERSION="${KERNEL_VERSION:-6.6.51}"
KERNEL_MAJOR="${KERNEL_VERSION%%.*}"
KERNEL_URL="https://cdn.kernel.org/pub/linux/kernel/v${KERNEL_MAJOR}.x/linux-${KERNEL_VERSION}.tar.xz"

# Colors for output
RED='\033[0;31m'
GREEN='\033[0;32m'
YELLOW='\033[1;33m'
NC='\033[0m' # No Color

log() { echo -e "${GREEN}[+]${NC} $*"; }
warn() { echo -e "${YELLOW}[!]${NC} $*"; }
error() { echo -e "${RED}[✗]${NC} $*"; exit 1; }

check_dependencies() {
    log "Checking build dependencies..."
    local deps=(gcc make flex bison bc perl)
    local missing=()
    
    for dep in "${deps[@]}"; do
        if ! command -v "$dep" &>/dev/null; then
            missing+=("$dep")
        fi
    done
    
    if [[ ${#missing[@]} -gt 0 ]]; then
        error "Missing dependencies: ${missing[*]}"
    fi
    
    # Check for headers
    if [[ ! -f /usr/include/libelf.h ]] && [[ ! -f /usr/include/elfutils/libelf.h ]]; then
        warn "libelf-dev might be missing (needed for BTF)"
    fi
}

download_kernel() {
    log "Downloading Linux kernel ${KERNEL_VERSION}..."
    
    mkdir -p "$BUILD_DIR"
    cd "$BUILD_DIR"
    
    if [[ -d "linux-${KERNEL_VERSION}" ]]; then
        log "Kernel source already exists, skipping download"
        return
    fi
    
    local tarball="linux-${KERNEL_VERSION}.tar.xz"
    if [[ ! -f "$tarball" ]]; then
        curl -L -o "$tarball" "$KERNEL_URL"
    fi
    
    log "Extracting kernel source..."
    tar xf "$tarball"
}

create_config() {
    log "Creating minimal microVM kernel config..."
    
    cd "${BUILD_DIR}/linux-${KERNEL_VERSION}"
    
    # Start with a minimal config
    make allnoconfig
    
    # Apply microVM-specific options
    cat >> .config << 'EOF'
# Basic system
CONFIG_64BIT=y
CONFIG_SMP=y
CONFIG_NR_CPUS=128
CONFIG_PREEMPT_VOLUNTARY=y
CONFIG_HIGH_RES_TIMERS=y
CONFIG_NO_HZ_IDLE=y
CONFIG_HZ_100=y

# PVH boot support (direct kernel boot)
CONFIG_PVH=y
CONFIG_XEN_PVH=y

# KVM guest support
CONFIG_HYPERVISOR_GUEST=y
CONFIG_PARAVIRT=y
CONFIG_KVM_GUEST=y
CONFIG_PARAVIRT_CLOCK=y
CONFIG_PARAVIRT_SPINLOCKS=y

# Memory
CONFIG_MEMORY_HOTPLUG=y
CONFIG_MEMORY_BALLOON=y
CONFIG_VIRTIO_BALLOON=y
CONFIG_BALLOON_COMPACTION=y

# Block devices
CONFIG_BLOCK=y
CONFIG_BLK_DEV=y
CONFIG_VIRTIO_BLK=y

# Networking
CONFIG_NET=y
CONFIG_PACKET=y
CONFIG_INET=y
CONFIG_VIRTIO_NET=y
CONFIG_VHOST_NET=y

# VirtIO core
CONFIG_VIRTIO=y
CONFIG_VIRTIO_MMIO=y
CONFIG_VIRTIO_PCI=y
CONFIG_VIRTIO_PCI_LEGACY=n
CONFIG_VIRTIO_CONSOLE=y

# Filesystems
CONFIG_EXT4_FS=y
CONFIG_PROC_FS=y
CONFIG_SYSFS=y
CONFIG_DEVTMPFS=y
CONFIG_DEVTMPFS_MOUNT=y
CONFIG_TMPFS=y
CONFIG_SQUASHFS=y
CONFIG_SQUASHFS_ZSTD=y

# TTY/Serial (for console)
CONFIG_TTY=y
CONFIG_VT=n
CONFIG_SERIAL_8250=y
CONFIG_SERIAL_8250_CONSOLE=y
CONFIG_SERIAL_8250_NR_UARTS=4
CONFIG_SERIAL_8250_RUNTIME_UARTS=4

# Minimal character devices
CONFIG_UNIX98_PTYS=y
CONFIG_DEVMEM=y

# Init
CONFIG_BINFMT_ELF=y
CONFIG_BINFMT_SCRIPT=y

# Crypto (minimal for boot)
CONFIG_CRYPTO=y
CONFIG_CRYPTO_CRC32C_INTEL=y

# Disable unnecessary features
CONFIG_MODULES=n
CONFIG_PRINTK=y
CONFIG_BUG=y
CONFIG_DEBUG_INFO=n
CONFIG_KALLSYMS=n
CONFIG_FTRACE=n
CONFIG_PROFILING=n
CONFIG_DEBUG_KERNEL=n

# 9P for host filesystem sharing
CONFIG_NET_9P=y
CONFIG_NET_9P_VIRTIO=y
CONFIG_9P_FS=y

# Compression support for initrd
CONFIG_RD_GZIP=y
CONFIG_RD_ZSTD=y

# Disable legacy/unused
CONFIG_USB_SUPPORT=n
CONFIG_SOUND=n
CONFIG_INPUT=n
CONFIG_SERIO=n
CONFIG_HW_RANDOM=y
CONFIG_HW_RANDOM_VIRTIO=y
CONFIG_DRM=n
CONFIG_FB=n
CONFIG_AGP=n
CONFIG_ACPI=n
CONFIG_PNP=n
CONFIG_WIRELESS=n
CONFIG_WLAN=n
CONFIG_RFKILL=n
CONFIG_BLUETOOTH=n
CONFIG_I2C=n
CONFIG_SPI=n
CONFIG_HWMON=n
CONFIG_THERMAL=n
CONFIG_WATCHDOG=n
CONFIG_MD=n
CONFIG_BT=n
CONFIG_NFS_FS=n
CONFIG_CIFS=n
CONFIG_SECURITY=n
CONFIG_AUDIT=n
EOF

    # Resolve any conflicts
    make olddefconfig
}

build_kernel() {
    log "Building kernel (this may take 5-15 minutes)..."
    
    cd "${BUILD_DIR}/linux-${KERNEL_VERSION}"
    
    # Parallel build using all cores
    local jobs
    jobs=$(nproc)
    
    make -j"$jobs" vmlinux
    
    # Copy output
    mkdir -p "$OUTPUT_DIR"
    cp vmlinux "${OUTPUT_DIR}/vmlinux"
    
    # Create a symlink to the versioned kernel
    ln -sf vmlinux "${OUTPUT_DIR}/vmlinux-${KERNEL_VERSION}"
}

show_stats() {
    local kernel="${OUTPUT_DIR}/vmlinux"
    
    if [[ -f "$kernel" ]]; then
        log "Kernel built successfully!"
        echo ""
        echo "  Path: $kernel"
        echo "  Size: $(du -h "$kernel" | cut -f1)"
        echo "  Kernel version: ${KERNEL_VERSION}"
        echo ""
        echo "To use with Volt:"
        echo "  volt-vmm --kernel ${kernel} --rootfs <rootfs> ..."
    else
        error "Kernel build failed - vmlinux not found"
    fi
}

# Main
main() {
    log "Building Volt microVM kernel v${KERNEL_VERSION}"
    echo ""
    
    check_dependencies
    download_kernel
    create_config
    build_kernel
    show_stats
}

main "$@"