arm WFE: add some userland examples

userland/arch/aarch64/freestanding/linux/wfe.S

@@ -0,0 +1,12 @@
+/* https://cirosantilli.com/linux-kernel-module-cheat#arm-wfe-and-sev-instructions */
+
+.text
+.global _start
+_start:
+asm_main_after_prologue:
+    wfe
+
+    /* exit */
+    mov x0, 0
+    mov x8, 93
+    svc 0

userland/arch/aarch64/freestanding/linux/wfe_wfe.S

@@ -0,0 +1,13 @@
+/* https://cirosantilli.com/linux-kernel-module-cheat#arm-wfe-and-sev-instructions */
+
+.text
+.global _start
+_start:
+asm_main_after_prologue:
+    wfe
+    wfe
+
+    /* exit */
+    mov x0, 0
+    mov x8, 93
+    svc 0

userland/arch/aarch64/inline_asm/wfe_sev.cpp

@@ -0,0 +1,29 @@
+// https://cirosantilli.com/linux-kernel-module-cheat#arm-wfe-and-sev-instructions
+
+#include <atomic>
+#include <iostream>
+#include <mutex>
+#include <thread>
+
+std::atomic_ulong done;
+
+void myfunc() {
+    unsigned long new_val = 1;
+    __asm__ __volatile__ ("wfe;wfe;" : "+r" (new_val) : :);
+    done.store(new_val);
+}
+
+int main(int argc, char **argv) {
+    bool do_sev = true;
+    if (argc > 1) {
+        do_sev = (argv[1][0] != '0');
+    }
+    done.store(0);
+    std::thread thread;
+    thread = std::thread(myfunc);
+    while (!done.load()) {
+        if (do_sev)
+            __asm__ __volatile__ ("sev");
+    }
+    thread.join();
+}