From b3624e40dfd672184b1c3b3d436e5a153de19948 Mon Sep 17 00:00:00 2001
From: Igor Velkov <325961+iav@users.noreply.github.com>
Date: Fri, 23 Jan 2026 00:03:07 +0200
Subject: [PATCH] extension: add arm64-compat-vdso for 32-bit vDSO support

Add extension that enables 32-bit compat vDSO for arm64 kernels.

Without vDSO, 32-bit applications using gettimeofday() syscall heavily
suffer significant performance penalty on arm64 systems.

The extension:
- Adds gcc-arm-linux-gnueabi as build dependency
- Sets CROSS_COMPILE_COMPAT for kernel make
- Enables CONFIG_COMPAT_VDSO in kernel config

Enable with: ENABLE_EXTENSIONS=arm64-compat-vdso

Refs: https://github.com/armbian/build/issues/9216

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
---
 extensions/arm64-compat-vdso.sh | 54 +++++++++++++++++++++++++++++++++
 1 file changed, 54 insertions(+)
 create mode 100644 extensions/arm64-compat-vdso.sh

diff --git a/extensions/arm64-compat-vdso.sh b/extensions/arm64-compat-vdso.sh
new file mode 100644
index 0000000000..69857fce4a
--- /dev/null
+++ b/extensions/arm64-compat-vdso.sh
@@ -0,0 +1,54 @@
+# Enable 32-bit compat vDSO for arm64 kernels with GCC or clang.
+# Requirements:
+# - arm64 build target (ARCH=arm64, ARCHITECTURE=arm64).
+# - For GCC builds: a 32-bit ARM cross-compiler (default prefix arm-linux-gnueabi-),
+#   available as ${CROSS_COMPILE_COMPAT}gcc; install gcc-arm-linux-gnueabi or set CROSS_COMPILE_COMPAT.
+# - For clang builds: clang present; compat vDSO is built via clang --target=arm-linux-gnueabi.
+
+function extension_prepare_config__arm64_compat_vdso() {
+	if [[ "${ARCH}" != "arm64" || "${ARCHITECTURE}" != "arm64" ]]; then
+		exit_with_error "arm64-only extension: ARCH=${ARCH} ARCHITECTURE=${ARCHITECTURE}"
+	fi
+}
+
+function add_host_dependencies__arm64_compat_vdso() {
+	if [[ "${KERNEL_COMPILER}" == "clang" ]]; then
+		EXTRA_BUILD_DEPS+=" clang "
+	else
+		EXTRA_BUILD_DEPS+=" gcc-arm-linux-gnueabi "
+	fi
+}
+
+function host_dependencies_ready__arm64_compat_vdso() {
+	if [[ "${KERNEL_COMPILER}" == "clang" ]]; then
+		return 0
+	fi
+
+	local compat_gcc_prefix="${CROSS_COMPILE_COMPAT:-"arm-linux-gnueabi-"}"
+	if ! command -v "${compat_gcc_prefix}gcc" >/dev/null 2>&1; then
+		exit_with_error "Missing 32-bit compiler '${compat_gcc_prefix}gcc' for COMPAT_VDSO; install gcc-arm-linux-gnueabi or set CROSS_COMPILE_COMPAT"
+	fi
+}
+
+function custom_kernel_make_params__arm64_compat_vdso() {
+	if [[ "${KERNEL_COMPILER}" == "clang" ]]; then
+		return 0
+	fi
+
+	local compat_gcc_compiler="${CROSS_COMPILE_COMPAT:-"arm-linux-gnueabi-"}"
+	common_make_params_quoted+=("CROSS_COMPILE_COMPAT=${compat_gcc_compiler}")
+	display_alert "arm64-compat-vdso" "Adding CROSS_COMPILE_COMPAT=${compat_gcc_compiler}" "info"
+}
+
+function custom_kernel_config__arm64_compat_vdso() {
+	local kconfig_hit=""
+
+	opts_y+=("COMPAT" "COMPAT_VDSO" "ARM64_32BIT_EL0")
+
+	if [[ -f .config ]]; then
+		kconfig_hit="$(grep -R -n -m1 "COMPAT_VDSO" arch/arm64 Kconfig* 2>/dev/null || true)"
+		if [[ -z "${kconfig_hit}" ]]; then
+			exit_with_error "Selected kernel tree lacks COMPAT_VDSO support for arm64"
+		fi
+	fi
+}