boot-qemu.sh: Support booting ARMv7 kernels under KVM on AArch64 hosts

boot-qemu.sh

@@ -168,7 +168,19 @@ function setup_qemu_args() {
                 -machine "virt${HIGHMEM}"
                 -no-reboot
             )
-            QEMU=(qemu-system-arm)
+            # It is possible to boot ARMv7 kernels under KVM on AArch64 hosts,
+            # if it is supported. ARMv7 KVM support was ripped out of the
+            # kernel in 5.7 so we don't even bother checking.
+            if [[ "$(uname -m)" = "aarch64" && -e /dev/kvm ]] && ${KVM} &&
+                "${BASE}"/utils/aarch64_32_bit_el1_supported; then
+                QEMU_ARCH_ARGS+=(
+                    -cpu "host,aarch64=off"
+                    -enable-kvm
+                )
+                QEMU=(qemu-system-aarch64)
+            else
+                QEMU=(qemu-system-arm)
+            fi
             ;;
 
         arm64 | arm64be)

utils/Makefile

@@ -0,0 +1,15 @@
+ifneq ($(shell uname -m),aarch64)
+CROSS_COMPILE = aarch64-linux-gnu-
+endif
+
+CC = $(CROSS_COMPILE)gcc
+STRIP = $(CROSS_COMPILE)strip
+
+aarch64_32_bit_el1_supported: aarch64_32_bit_el1_supported.c
+	$(CC) -O2 -static -std=c17 -Wall -Wextra -Wpedantic -o $@ $^
+	$(STRIP) -s $@
+
+clean:
+	rm -fr aarch64_32_bit_el1_supported
+
+all: aarch64_32_bit_el1_supported

utils/aarch64_32_bit_el1_supported.c

@@ -0,0 +1,31 @@
+#include <errno.h>	/* errno for perror() */
+#include <linux/kvm.h>	/* KVM_CHECK_EXTENSION, KVM_CAP_ARM_EL1_32BIT */
+#include <fcntl.h>	/* open() */
+#include <stdio.h>	/* perror() */
+#include <sys/ioctl.h>	/* ioctl() */
+#include <unistd.h>	/* close() */
+
+int main(void)
+{
+	int fd, ret;
+
+	fd = open("/dev/kvm", O_RDWR);
+	if (fd < 0) {
+		perror("Failed to open /dev/kvm");
+		return -errno;
+	}
+
+	ret = ioctl(fd, KVM_CHECK_EXTENSION, KVM_CAP_ARM_EL1_32BIT);
+	if (ret < 0) {
+		perror("Error checking /dev/kvm for 32-bit EL1 support");
+		ret = 0;
+	}
+
+	close(fd);
+
+	/*
+	 * KVM_CHECK_EXTENSION returns 1 for supported, 0 for unsupported so
+	 * invert it to match typical success/fail codes in programs.
+	 */
+	return !ret;
+}