wfe_ldrex_strex.cpp

README.adoc

@@ -18634,8 +18634,6 @@ However, likely no implementation likely does (TODO confirm), since:
 
 and power consumption is key in ARM applications.
 
-SEV is not the only thing that can wake up a WFE, it is only an explicit software way to do it. Notably, global monitor operations on memory accesses of regions marked by LDAXR and STLXR instructions can also wake up a WFE sleeping core. This is done to allow spinlocks opens to automatically wake up WFE sleeping cores at free time without the need for a explicit SEV.
-
 Quotes for the above <<armarm8-db>> G1.18.1 "Wait For Event and Send Event":
 
 ____
@@ -18693,6 +18691,20 @@ The following Raspberry Pi bibliography helped us get this sample up and running
 
 For how userland spinlocks and mutexes are implemented see <<userland-mutex-implementation>>.
 
+====== ARM WFE global monitor events
+
+link:userland/arch/aarch64/inline_asm/wfe_ldxr_stxr.cpp[]
+
+SEV is not the only thing that can wake up a WFE, it is only an explicit software way to do it.
+
+Notably, global monitor operations on memory accesses of regions marked by <<arm-ldxr-and-stxr-instructions,LDAXR and STLXR instructions>> can also wake up a WFE sleeping core.
+
+This is done to allow spinlocks opens to automatically wake up WFE sleeping cores at free time without the need for a explicit SEV.
+
+In the shown in the `wfe_ldrex_strex.cpp` example, which can only terminate in gem5 user mode simulation because due to this event.
+
+Note that that program still terminates when running on top of the Linux kernel as explained at: <<wfe-from-userland>>.
+
 ====== WFE from userland
 
 WFE and SEV are usable from userland, and are part of an efficient spinlock implementation (which userland should arguably stay away from and rather use the <<futex-system-call>> which allow for non busy sleep instead), which maybe is not something that userland should ever tho and just stick to mutexes?

userland/arch/aarch64/inline_asm/wfe_ldxr_stxr.cpp

@@ -0,0 +1,35 @@
+// https://cirosantilli.com/linux-kernel-module-cheat#arm-wfe-global-monitor-events
+
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+#include <atomic>
+#include <iostream>
+#include <mutex>
+#include <thread>
+
+#include <lkmc/futex.h>
+
+std::atomic_ulong done;
+int futex = 1;
+
+void myfunc() {
+    __asm__ __volatile__ ("ldxr x0, [%0];wfe;wfe" : : "r" (&futex) : "x0");
+    done.store(futex);
+}
+
+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__ ("mov x0, 1;stxr w1, x0, [%0]" : : "r" (&futex) : "x0", "x1");
+        }
+    }
+    thread.join();
+}