[Qemu-ppc] [RFC PATCH] Implement qemu_thread_yield for posix, use it in mttcg to handle EXCP_YIELD

[Nicholas Piggin]

This is a bit of proof of concept in case mttcg becomes more important
yield could be handled like this. You can have by accident or deliberately
force vCPUs onto the same physical CPU and cause inversion issues when the
lock holder was preempted by the waiter. This is lightly tested but not
to the point of measuring performance difference.

I really consider the previous confer/prod patches more important just to
provide a more complete guest environment and better test coverage, than
performance, but maybe someone wants to persue this.

Thanks,
Nick
---
 cpus.c                   |  6 ++++++
 hw/ppc/spapr_hcall.c     | 14 +++++++-------
 include/qemu/thread.h    |  1 +
 util/qemu-thread-posix.c |  5 +++++
 util/qemu-thread-win32.c |  4 ++++
 5 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/cpus.c b/cpus.c
index 927a00aa90..f036e062d9 100644
--- a/cpus.c
+++ b/cpus.c
@@ -1760,6 +1760,12 @@ static void *qemu_tcg_cpu_thread_fn(void *arg)
                 qemu_mutex_unlock_iothread();
                 cpu_exec_step_atomic(cpu);
                 qemu_mutex_lock_iothread();
+                break;
+            case EXCP_YIELD:
+                qemu_mutex_unlock_iothread();
+                qemu_thread_yield();
+                qemu_mutex_lock_iothread();
+                break;
             default:
                 /* Ignore everything else? */
                 break;
diff --git a/hw/ppc/spapr_hcall.c b/hw/ppc/spapr_hcall.c
index 57c1ee0fe1..9c24a64dfe 100644
--- a/hw/ppc/spapr_hcall.c
+++ b/hw/ppc/spapr_hcall.c
@@ -1162,13 +1162,13 @@ static target_ulong h_confer(PowerPCCPU *cpu, 
SpaprMachineState *spapr,
             return H_SUCCESS;
         }
 
-        /*
-         * The targeted confer does not do anything special beyond yielding
-         * the current vCPU, but even this should be better than nothing.
-         * At least for single-threaded tcg, it gives the target a chance to
-         * run before we run again. Multi-threaded tcg does not really do
-         * anything with EXCP_YIELD yet.
-         */
+       /*
+        * The targeted confer does not do anything special beyond yielding
+        * the current vCPU, but even this should be better than nothing.
+        * For single-threaded tcg, it gives the target a chance to run
+        * before we run again, multi-threaded tcg will yield the CPU to
+        * another vCPU.
+        */
     }
 
     cs->exception_index = EXCP_YIELD;
diff --git a/include/qemu/thread.h b/include/qemu/thread.h
index 55d83a907c..8525b0a70a 100644
--- a/include/qemu/thread.h
+++ b/include/qemu/thread.h
@@ -160,6 +160,7 @@ void qemu_thread_get_self(QemuThread *thread);
 bool qemu_thread_is_self(QemuThread *thread);
 void qemu_thread_exit(void *retval);
 void qemu_thread_naming(bool enable);
+void qemu_thread_yield(void);
 
 struct Notifier;
 /**
diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index 1bf5e65dea..91b12a1082 100644
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -573,3 +573,8 @@ void *qemu_thread_join(QemuThread *thread)
     }
     return ret;
 }
+
+void qemu_thread_yield(void)
+{
+    pthread_yield();
+}
diff --git a/util/qemu-thread-win32.c b/util/qemu-thread-win32.c
index 572f88535d..72fe406bef 100644
--- a/util/qemu-thread-win32.c
+++ b/util/qemu-thread-win32.c
@@ -442,3 +442,7 @@ bool qemu_thread_is_self(QemuThread *thread)
 {
     return GetCurrentThreadId() == thread->tid;
 }
+
+void qemu_thread_yield(void)
+{
+}
-- 
2.20.1