selftests/bpf: Enable cross platform testing for vmtest

Add support cross platform testing for vmtest. The variable $ARCH in the
current script is platform semantics, not kernel semantics. Rename it to
$PLATFORM so that we can easily use $ARCH in cross-compilation. And drop
`set -u` unbound variable check as we will use CROSS_COMPILE env
variable. For now, Using PLATFORM= and CROSS_COMPILE= options will
enable cross platform testing:

  PLATFORM=<platform> CROSS_COMPILE=<toolchain> vmtest.sh -- ./test_progs
Tested-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Link: https://lore.kernel.org/r/20240905081401.1894789-7-pulehui@huaweicloud.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/vmtest.sh

 #!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
 
-set -u
 set -e
 
 # This script currently only works for the following platforms,
@@ -9,25 +8,31 @@ set -e
 # available only for these architectures. We can also specify
 # the local rootfs image generated by the following script:
 # https://github.com/libbpf/ci/blob/main/rootfs/mkrootfs_debian.sh
-ARCH="$(uname -m)"
-case "${ARCH}" in
+PLATFORM="${PLATFORM:-$(uname -m)}"
+case "${PLATFORM}" in
 s390x)
 	QEMU_BINARY=qemu-system-s390x
 	QEMU_CONSOLE="ttyS1"
-	QEMU_FLAGS=(-smp 2)
+	HOST_FLAGS=(-smp 2 -enable-kvm)
+	CROSS_FLAGS=(-smp 2)
 	BZIMAGE="arch/s390/boot/vmlinux"
+	ARCH="s390"
 	;;
 x86_64)
 	QEMU_BINARY=qemu-system-x86_64
 	QEMU_CONSOLE="ttyS0,115200"
-	QEMU_FLAGS=(-cpu host -smp 8)
+	HOST_FLAGS=(-cpu host -enable-kvm -smp 8)
+	CROSS_FLAGS=(-smp 8)
 	BZIMAGE="arch/x86/boot/bzImage"
+	ARCH="x86"
 	;;
 aarch64)
 	QEMU_BINARY=qemu-system-aarch64
 	QEMU_CONSOLE="ttyAMA0,115200"
-	QEMU_FLAGS=(-M virt,gic-version=3 -cpu host -smp 8)
+	HOST_FLAGS=(-M virt,gic-version=3 -cpu host -enable-kvm -smp 8)
+	CROSS_FLAGS=(-M virt,gic-version=3 -cpu cortex-a76 -smp 8)
 	BZIMAGE="arch/arm64/boot/Image"
+	ARCH="arm64"
 	;;
 *)
 	echo "Unsupported architecture"
@@ -41,7 +46,7 @@ ROOTFS_IMAGE="root.img"
 OUTPUT_DIR="$HOME/.bpf_selftests"
 KCONFIG_REL_PATHS=("tools/testing/selftests/bpf/config"
 	"tools/testing/selftests/bpf/config.vm"
-	"tools/testing/selftests/bpf/config.${ARCH}")
+	"tools/testing/selftests/bpf/config.${PLATFORM}")
 INDEX_URL="https://raw.githubusercontent.com/libbpf/ci/master/INDEX"
 NUM_COMPILE_JOBS="$(nproc)"
 LOG_FILE_BASE="$(date +"bpf_selftests.%Y-%m-%d_%H-%M-%S")"
@@ -61,6 +66,10 @@ tools/testing/selftests/bpf. e.g:
 If no command is specified and a debug shell (-s) is not requested,
 "${DEFAULT_COMMAND}" will be run by default.
 
+Using PLATFORM= and CROSS_COMPILE= options will enable cross platform testing:
+
+  PLATFORM=<platform> CROSS_COMPILE=<toolchain> $0 -- ./test_progs -t test_lsm
+
 If you build your kernel using KBUILD_OUTPUT= or O= options, these
 can be passed as environment variables to the script:
 
@@ -100,7 +109,7 @@ newest_rootfs_version()
 {
 	{
 	for file in "${!URLS[@]}"; do
-		if [[ $file =~ ^"${ARCH}"/libbpf-vmtest-rootfs-(.*)\.tar\.zst$ ]]; then
+		if [[ $file =~ ^"${PLATFORM}"/libbpf-vmtest-rootfs-(.*)\.tar\.zst$ ]]; then
 			echo "${BASH_REMATCH[1]}"
 		fi
 	done
@@ -112,7 +121,7 @@ download_rootfs()
 	populate_url_map
 
 	local rootfsversion="$(newest_rootfs_version)"
-	local file="${ARCH}/libbpf-vmtest-rootfs-$rootfsversion.tar.zst"
+	local file="${PLATFORM}/libbpf-vmtest-rootfs-$rootfsversion.tar.zst"
 
 	if [[ ! -v URLS[$file] ]]; then
 		echo "$file not found" >&2
@@ -253,12 +262,17 @@ EOF
 		exit 1
 	fi
 
+	if [[ "${PLATFORM}" != "$(uname -m)" ]]; then
+		QEMU_FLAGS=("${CROSS_FLAGS[@]}")
+	else
+		QEMU_FLAGS=("${HOST_FLAGS[@]}")
+	fi
+
 	${QEMU_BINARY} \
 		-nodefaults \
 		-display none \
 		-serial mon:stdio \
 		"${QEMU_FLAGS[@]}" \
-		-enable-kvm \
 		-m 4G \
 		-drive file="${rootfs_img}",format=raw,index=1,media=disk,if=virtio,cache=none \
 		-kernel "${kernel_bzimage}" \
@@ -389,6 +403,11 @@ main()
 
 	trap 'catch "$?"' EXIT
 
+	if [[ "${PLATFORM}" != "$(uname -m)" ]] && [[ -z "${CROSS_COMPILE}" ]]; then
+		echo "Cross-platform testing needs to specify CROSS_COMPILE"
+		exit 1
+	fi
+
 	if [[ $# -eq 0  && "${debug_shell}" == "no" ]]; then
 		echo "No command specified, will run ${DEFAULT_COMMAND} in the vm"
 	else
@@ -396,7 +415,8 @@ main()
 	fi
 
 	local kconfig_file="${OUTPUT_DIR}/latest.config"
-	local make_command="make -j ${NUM_COMPILE_JOBS} KCONFIG_CONFIG=${kconfig_file}"
+	local make_command="make ARCH=${ARCH} CROSS_COMPILE=${CROSS_COMPILE} \
+			    -j ${NUM_COMPILE_JOBS} KCONFIG_CONFIG=${kconfig_file}"
 
 	# Figure out where the kernel is being built.
 	# O takes precedence over KBUILD_OUTPUT.