Subject: [kvm-unit-tests PATCH v9 19/31] powerpc: Avoid using larx/stcx. in spinlocks when only one CPU is running

From: Nicholas Piggin <[email-masked]>
Date: Sat, 4 May 2024 22:28:25 +1000

The test harness uses spinlocks if they are implemented with larx/stcx.
it can prevent some test scenarios such as testing migration of a
reservation.

Signed-off-by: Nicholas Piggin <[email-masked]>
---
 lib/powerpc/asm/smp.h    |  1 +
 lib/powerpc/smp.c        |  5 +++++
 lib/powerpc/spinlock.c   | 29 +++++++++++++++++++++++++++++
 lib/ppc64/asm/spinlock.h |  7 ++++++-
 powerpc/Makefile.common  |  1 +
 5 files changed, 42 insertions(+), 1 deletion(-)
 create mode 100644 lib/powerpc/spinlock.c

diff --git a/lib/powerpc/asm/smp.h b/lib/powerpc/asm/smp.h
index c45988bfa..66188b9dd 100644
--- a/lib/powerpc/asm/smp.h
+++ b/lib/powerpc/asm/smp.h
@@ -15,6 +15,7 @@ struct cpu {
 
 extern int nr_cpus_present;
 extern int nr_cpus_online;
+extern bool multithreaded;
 extern struct cpu cpus[];
 
 register struct cpu *__current_cpu asm("r13");
diff --git a/lib/powerpc/smp.c b/lib/powerpc/smp.c
index 27b169841..73c0ef214 100644
--- a/lib/powerpc/smp.c
+++ b/lib/powerpc/smp.c
@@ -276,6 +276,8 @@ static void start_each_secondary(int fdtnode, u64 regval __unused, void *info)
 	start_core(fdtnode, datap->entry);
 }
 
+bool multithreaded = false;
+
 /*
  * Start all stopped cpus on the guest at entry with register 3 set to r3
  * We expect that we come in with only one thread currently started
@@ -290,6 +292,7 @@ bool start_all_cpus(secondary_entry_fn entry)
 
 	assert(nr_cpus_online == 1);
 	assert(nr_started == 1);
+	multithreaded = true;
 	ret = dt_for_each_cpu_node(start_each_secondary, &data);
 	assert(ret == 0);
 	assert(nr_started == nr_cpus_present);
@@ -361,10 +364,12 @@ static void wait_each_secondary(int fdtnode, u64 regval __unused, void *info)
 
 void stop_all_cpus(void)
 {
+	assert(multithreaded);
 	while (nr_cpus_online > 1)
 		cpu_relax();
 
 	dt_for_each_cpu_node(wait_each_secondary, NULL);
 	mb();
 	nr_started = 1;
+	multithreaded = false;
 }
diff --git a/lib/powerpc/spinlock.c b/lib/powerpc/spinlock.c
new file mode 100644
index 000000000..623a1f2c1
--- /dev/null
+++ b/lib/powerpc/spinlock.c
@@ -0,0 +1,29 @@
+/* SPDX-License-Identifier: LGPL-2.0 */
+#include <asm/spinlock.h>
+#include <asm/smp.h>
+
+/*
+ * Skip the atomic when single-threaded, which helps avoid larx/stcx. in
+ * the harness when testing tricky larx/stcx. sequences (e.g., migration
+ * vs reservation).
+ */
+void spin_lock(struct spinlock *lock)
+{
+	if (!multithreaded) {
+		assert(lock->v == 0);
+		lock->v = 1;
+	} else {
+		while (__sync_lock_test_and_set(&lock->v, 1))
+			;
+	}
+}
+
+void spin_unlock(struct spinlock *lock)
+{
+	assert(lock->v == 1);
+	if (!multithreaded) {
+		lock->v = 0;
+	} else {
+		__sync_lock_release(&lock->v);
+	}
+}
diff --git a/lib/ppc64/asm/spinlock.h b/lib/ppc64/asm/spinlock.h
index f59eed191..b952386da 100644
--- a/lib/ppc64/asm/spinlock.h
+++ b/lib/ppc64/asm/spinlock.h
@@ -1,6 +1,11 @@
 #ifndef _ASMPPC64_SPINLOCK_H_
 #define _ASMPPC64_SPINLOCK_H_
 
-#include <asm-generic/spinlock.h>
+struct spinlock {
+	unsigned int v;
+};
+
+void spin_lock(struct spinlock *lock);
+void spin_unlock(struct spinlock *lock);
 
 #endif /* _ASMPPC64_SPINLOCK_H_ */
diff --git a/powerpc/Makefile.common b/powerpc/Makefile.common
index b98f71c2f..1ee9c25d6 100644
--- a/powerpc/Makefile.common
+++ b/powerpc/Makefile.common
@@ -49,6 +49,7 @@ cflatobjs += lib/powerpc/rtas.o
 cflatobjs += lib/powerpc/processor.o
 cflatobjs += lib/powerpc/handlers.o
 cflatobjs += lib/powerpc/smp.o
+cflatobjs += lib/powerpc/spinlock.o
 
 OBJDIRS += lib/powerpc
 
-- 
2.43.0