init: reboot after reporting the workload exit code

KRUN_EXIT_CODE_IOCTL is used only as a transfer workload exit code
Guest, no generic VMM shutdown mechanism is provided.

In the past init logged the workload exit code and simply returned.
On platforms where there is no shutdown/reset path to wake VMM later, the guest
Can continue running after the workload exits.

Keep ioctl for exit code transmission once reported by init
Workload exit code requesting kernel-mediated reboot/reset for VMM
Still exit via existing platform closure path.

On x86 guests, it is important to use the reboot/reset path as powering off may cause
Fallback to a stopped state, which does not trigger libkrun's VM exit path.

Add end-to-end tests to verify that the runner exits with the workload
Exit code.

Signed-off-by: Zewei Yang <yangzewei@loongson.cn>

Author: yzewei

init/init.c

@@ -14,6 +14,7 @@
 #include <net/if.h>
 #include <sys/ioctl.h>
 #include <sys/mount.h>
+#include <sys/reboot.h>
 #include <sys/resource.h>
 #include <sys/socket.h>
 #include <sys/stat.h>
@@ -1003,6 +1004,20 @@ void set_exit_code(int code)
     close(fd);
 }
 
+static void shutdown_vm(void)
+{
+    sync();
+
+    /*
+     * Use a kernel-mediated reboot here rather than RB_POWER_OFF.
+     * On x86 guests, RB_POWER_OFF may fall back to halt, which does not
+     * trigger libkrun's VM exit path, while reboot/reset does.
+     */
+    if (reboot(RB_AUTOBOOT) < 0) {
+        printf("Error: reboot() failed: %s\n", strerror(errno));
+    }
+}
+
 int try_mount(const char *source, const char *target, const char *fstype,
               unsigned long mountflags, const void *data)
 {
@@ -1241,13 +1256,12 @@ int main(int argc, char **argv)
             // Not the first child, ignore it.
         };
 
-        // The workload's entrypoint has exited, record its exit code and exit
-        // ourselves.
         if (WIFEXITED(status)) {
             set_exit_code(WEXITSTATUS(status));
         } else if (WIFSIGNALED(status)) {
             set_exit_code(WTERMSIG(status) + 128);
         }
+        shutdown_vm();
     }
 
     return 0;