cpus: clear exit_request in qemu_process_cpu_events

Make the code common to all accelerators: after seeing cpu->exit_request
set to true, accelerator code needs to reach qemu_process_cpu_events_common().

So for the common cases where they use qemu_process_cpu_events(), go ahead and
clear it in there.  Note that the cheap qatomic_set() is enough because
at this point the thread has taken the BQL; qatomic_set_mb() is not needed.
In particular, this is the ordering of the communication between
I/O and vCPU threads is always the same.

In the I/O thread:

(a) store other memory locations that will be checked if cpu->exit_request
    or cpu->interrupt_request is 1 (for example cpu->stop or cpu->work_list
    for cpu->exit_request)

(b) cpu_exit(): store-release cpu->exit_request, or
(b) cpu_interrupt(): store-release cpu->interrupt_request

>>> at this point, cpu->halt_cond is broadcast and the BQL released

(c) do the accelerator-specific kick (e.g. write icount_decr for TCG,
    pthread_kill for KVM, etc.)

In the vCPU thread instead the opposite order is respected:

(c) the accelerator's execution loop exits thanks to the kick

(b) then the inner execution loop checks cpu->interrupt_request
    and cpu->exit_request.  If needed cpu->interrupt_request is
    converted into cpu->exit_request when work is needed outside
    the execution loop.

(a) then the other memory locations are checked.  Some may need to
    be read under the BQL, but the vCPU thread may also take other
    locks (e.g. for queued work items) or none at all.

qatomic_set_mb() would only be needed if the halt sleep was done
outside the BQL (though in that case, cpu->exit_request probably
would be replaced by a QemuEvent or something like that).

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

accel/kvm/kvm-all.c

@@ -3155,7 +3155,6 @@ int kvm_cpu_exec(CPUState *cpu)
     trace_kvm_cpu_exec();
 
     if (kvm_arch_process_async_events(cpu)) {
-        qatomic_set(&cpu->exit_request, 0);
         return EXCP_HLT;
     }
 
@@ -3345,7 +3344,6 @@ int kvm_cpu_exec(CPUState *cpu)
         vm_stop(RUN_STATE_INTERNAL_ERROR);
     }
 
-    qatomic_set(&cpu->exit_request, 0);
     return ret;
 }
 

accel/tcg/cpu-exec.c

@@ -872,7 +872,6 @@ static inline bool cpu_handle_interrupt(CPUState *cpu,
      * The corresponding store-release is in cpu_exit.
      */
     if (unlikely(qatomic_load_acquire(&cpu->exit_request)) || icount_exit_request(cpu)) {
-        qatomic_set(&cpu->exit_request, 0);
         if (cpu->exception_index == -1) {
             cpu->exception_index = EXCP_INTERRUPT;
         }

accel/tcg/tcg-accel-ops-rr.c

@@ -293,8 +293,13 @@ static void *rr_cpu_thread_fn(void *arg)
         /* Does not need a memory barrier because a spurious wakeup is okay.  */
         qatomic_set(&rr_current_cpu, NULL);
 
-        if (cpu && qatomic_read(&cpu->exit_request)) {
-            qatomic_set_mb(&cpu->exit_request, 0);
+        if (cpu) {
+            /*
+             * This could even reset exit_request for all CPUs, but in practice
+             * races between CPU exits and changes to "cpu" are so rare that
+             * there's no advantage in doing so.
+             */
+            qatomic_set(&cpu->exit_request, false);
         }
 
         if (icount_enabled() && all_cpu_threads_idle()) {

accel/tcg/tcg-accel-ops.c

@@ -82,8 +82,6 @@ int tcg_cpu_exec(CPUState *cpu)
     ret = cpu_exec(cpu);
     cpu_exec_end(cpu);
 
-    qatomic_set_mb(&cpu->exit_request, 0);
-
     return ret;
 }
 

accel/tcg/user-exec.c

@@ -54,6 +54,7 @@ void qemu_cpu_kick(CPUState *cpu)
 
 void qemu_process_cpu_events(CPUState *cpu)
 {
+    qatomic_set(&cpu->exit_request, false);
     process_queued_cpu_work(cpu);
 }
 

system/cpus.c

@@ -463,6 +463,7 @@ void qemu_process_cpu_events(CPUState *cpu)
 {
     bool slept = false;
 
+    qatomic_set(&cpu->exit_request, false);
     while (cpu_thread_is_idle(cpu)) {
         if (!slept) {
             slept = true;

target/i386/nvmm/nvmm-all.c

@@ -817,8 +817,6 @@ nvmm_vcpu_loop(CPUState *cpu)
     cpu_exec_end(cpu);
     bql_lock();
 
-    qatomic_set(&cpu->exit_request, false);
-
     return ret < 0;
 }
 

target/i386/whpx/whpx-all.c

@@ -2050,8 +2050,6 @@ static int whpx_vcpu_run(CPUState *cpu)
         whpx_last_vcpu_stopping(cpu);
     }
 
-    qatomic_set(&cpu->exit_request, false);
-
     return ret < 0;
 }