[Qemu-devel] [PATCH 3/9] tcg: introduce tcg_current_cpu

[Paolo Bonzini <pbonzini@redhat.com>]

This is already useful on Windows in order to remove tls.h, because
accesses to current_cpu are done from a different thread on that
platform.  It will be used on POSIX platforms as soon TCG stops using
signals to interrupt the execution of translated code.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 cpu-exec.c              | 14 +++++---------
 cpus.c                  |  5 +++--
 include/exec/exec-all.h |  1 +
 3 files changed, 9 insertions(+), 11 deletions(-)

diff --git a/cpu-exec.c b/cpu-exec.c
index 58144b4..2c3cb7d 100644
--- a/cpu-exec.c
+++ b/cpu-exec.c
@@ -382,6 +382,7 @@ static void cpu_handle_debug_exception(CPUState *cpu)
 /* main execution loop */
 
 volatile sig_atomic_t exit_request;
+CPUState *tcg_current_cpu;
 
 int cpu_exec(CPUState *cpu)
 {
@@ -405,15 +406,7 @@ int cpu_exec(CPUState *cpu)
     }
 
     current_cpu = cpu;
-
-    /* As long as current_cpu is null, up to the assignment just above,
-     * requests by other threads to exit the execution loop are expected to
-     * be issued using the exit_request global. We must make sure that our
-     * evaluation of the global value is performed past the current_cpu
-     * value transition point, which requires a memory barrier as well as
-     * an instruction scheduling constraint on modern architectures.  */
-    smp_mb();
-
+    atomic_mb_set(&tcg_current_cpu, cpu);
     rcu_read_lock();
 
     if (unlikely(exit_request)) {
@@ -614,5 +607,8 @@ int cpu_exec(CPUState *cpu)
 
     /* fail safe : never use current_cpu outside cpu_exec() */
     current_cpu = NULL;
+
+    /* Does not need atomic_mb_set because a spurious wakeup is okay.  */
+    atomic_set(&tcg_current_cpu, NULL);
     return ret;
 }
diff --git a/cpus.c b/cpus.c
index 8884278..ec8168c 100644
--- a/cpus.c
+++ b/cpus.c
@@ -663,8 +663,9 @@ static void cpu_handle_guest_debug(CPUState *cpu)
 
 static void cpu_signal(int sig)
 {
-    if (current_cpu) {
-        cpu_exit(current_cpu);
+    CPUState *cpu = atomic_mb_read(&tcg_current_cpu);
+    if (cpu) {
+        cpu_exit(cpu);
     }
     exit_request = 1;
 }
diff --git a/include/exec/exec-all.h b/include/exec/exec-all.h
index b3f900a..c92d434 100644
--- a/include/exec/exec-all.h
+++ b/include/exec/exec-all.h
@@ -346,6 +346,7 @@ tb_page_addr_t get_page_addr_code(CPUArchState *env1, target_ulong addr);
 extern int singlestep;
 
 /* cpu-exec.c */
+extern CPUState *tcg_current_cpu;
 extern volatile sig_atomic_t exit_request;
 
 #endif
-- 
2.4.3