Add msgsync to doorbell wakeup logic to avoid weak consistency bug

dcrowell77 · wghoffa · commit 51206826999d · 2019-01-21T08:55:21.000-06:00
POWER9 added a new sync mode called 'msgsync' that is required to avoid weak consistency issues when you are using doorbell (msgsnd) functions. See POWER ISA Section 5.9.2 for details, excerpt here: The ordering done by sync (and ptesync) provides the appearance of "causality" across a sequence of msgsnd instructions, as in the following example. "msgsnd->T1" means "msgsnd instruction target- ting thread T1". "<DHDI 0>" means "occurrence of Directed Hypervisor Doorbell interrupt caused by msgsnd executed on T0". On T0, register r1 is assumed to contain the value 1. T0 T1 T2 std r1,X <DHDI 0> <DHDI 1> sync msgsnd->T2 msgsync msgsnd->T1 ld r1,X In this example, T2's load from X must return 1. The change here adds the msgsync call to the code that executes any time we handle a doorbell interrupt. In addition there is a POWER9 DD2 errata that indicates we also require a lwsync to ensure consistency. Change-Id: Ib0f3571926d71efcbffa205093278e2a1d58df85 CQ: SW454611 Reviewed-on: http://rchgit01.rchland.ibm.com/gerrit1/70650 Tested-by: Jenkins Server <pfd-jenkins+hostboot@us.ibm.com> Tested-by: Jenkins OP Build CI <op-jenkins+hostboot@us.ibm.com> Tested-by: Jenkins OP HW <op-hw-jenkins+hostboot@us.ibm.com> Reviewed-by: William G. Hoffa <wghoffa@us.ibm.com>
diff --git a/src/include/arch/ppc.H b/src/include/arch/ppc.H
@@ -5,7 +5,7 @@
 /*                                                                        */
 /* OpenPOWER HostBoot Project                                             */
 /*                                                                        */
-/* Contributors Listed Below - COPYRIGHT 2011,2018                        */
+/* Contributors Listed Below - COPYRIGHT 2011,2019                        */
 /* [+] Google Inc.                                                        */
 /* [+] International Business Machines Corp.                              */
 /*                                                                        */
@@ -225,6 +225,20 @@ inline void eieio()
     asm volatile("eieio" ::: "memory");
 }
 
+ALWAYS_INLINE
+inline void msgsync()
+{
+    // See POWER ISA 5.9.2 for details
+    //asm volatile("msgsync" ::: "memory");
+    asm volatile(".long 0x7C0006EC"); // Our GCC doesn't support 'msgsync' yet
+    //  [011111 ///// ///// ///// 11011 10110/]
+
+    // There is a P9 DD2 workaround that a lwsync is also required
+    //  after a msgsync
+    asm volatile("lwsync" ::: "memory");
+}
+
+
 ALWAYS_INLINE
 inline uint64_t getHMER()
 {
diff --git a/src/kernel/syscall.C b/src/kernel/syscall.C
@@ -5,7 +5,7 @@
 /*                                                                        */
 /* OpenPOWER HostBoot Project                                             */
 /*                                                                        */
-/* Contributors Listed Below - COPYRIGHT 2010,2018                        */
+/* Contributors Listed Below - COPYRIGHT 2010,2019                        */
 /* [+] International Business Machines Corp.                              */
 /*                                                                        */
 /*                                                                        */
@@ -55,6 +55,12 @@ namespace KernelIpc
 extern "C"
 void kernel_execute_hype_doorbell()
 {
+    // Per POWER ISA Section 5.9.2, to avoid any weak consistency
+    //  issues we must use a msgsync instruction before consuming
+    //  any data set by a different thread following a doorbell
+    //  wakeup.
+    msgsync();
+
     task_t* t = TaskManager::getCurrentTask();
     doorbell_clear();
 
