vl: allow other threads to do qemu_system_vmstop_request

There patch protects vmstop_requested with a lock and introduces
qemu_system_vmstop_request_prepare.

Together with the new call to qemu_vmstop_requested in vm_start,
qemu_system_vmstop_request_prepare avoids a race where the VM could remain
stopped even though the iostatus of a block device has already been set
(for example).

qemu_system_vmstop_request_prepare however also lets the caller thread
delay observation of the state change until it has itself communicated
that change to the user.  This delay avoids any possibility of a wrong
reordering of the BLOCK_IO_ERROR event and the subsequent STOP event.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/cpus.c b/cpus.c
index 1ec3a9edd4..06da4e7665 100644
--- a/cpus.c
+++ b/cpus.c
@@ -1206,6 +1206,7 @@ void cpu_stop_current(void)
 int vm_stop(RunState state)
 {
     if (qemu_in_vcpu_thread()) {
+        qemu_system_vmstop_request_prepare();
         qemu_system_vmstop_request(state);
         /*
          * FIXME: should not return to device code in case