[Qemu-devel] [PATCH 3/3] configure: Don't fall back to gthread coroutine backend

[Peter Maydell]

The gthread coroutine backend is broken and does not produce a working
QEMU; it is only useful for some very limited debugging situations.
Clean up the backend selection logic in configure so that it now runs
"if on windows use windows; else prefer ucontext; else sigaltstack".

To do this we refactor the configure code to separate out "test
whether we have a working ucontext", "pick a default if user didn't
specify" and "validate that user didn't specify something invalid",
rather than having all three of these run together. We also have to
adjust the Makefile logic so it doesn't also encode an idea of the
default backend.

Signed-off-by: Peter Maydell <address@hidden>
---
 Makefile.objs |    6 +++---
 configure     |   67 +++++++++++++++++++++++++++++++++++++--------------------
 2 files changed, 47 insertions(+), 26 deletions(-)

diff --git a/Makefile.objs b/Makefile.objs
index f99841c..7f541a4 100644
--- a/Makefile.objs
+++ b/Makefile.objs
@@ -18,12 +18,12 @@ block-obj-y += qemu-coroutine.o qemu-coroutine-lock.o 
qemu-coroutine-io.o
 block-obj-y += qemu-coroutine-sleep.o
 ifeq ($(CONFIG_UCONTEXT_COROUTINE),y)
 block-obj-$(CONFIG_POSIX) += coroutine-ucontext.o
-else
+endif
 ifeq ($(CONFIG_SIGALTSTACK_COROUTINE),y)
 block-obj-$(CONFIG_POSIX) += coroutine-sigaltstack.o
-else
-block-obj-$(CONFIG_POSIX) += coroutine-gthread.o
 endif
+ifeq ($(CONFIG_GTHREAD_COROUTINE),y)
+block-obj-$(CONFIG_POSIX) += coroutine-gthread.o
 endif
 block-obj-$(CONFIG_WIN32) += coroutine-win32.o
 
diff --git a/configure b/configure
index d9dfde0..9f7f5cd 100755
--- a/configure
+++ b/configure
@@ -3052,31 +3052,55 @@ fi
 ##########################################
 # check and set a backend for coroutine
 
-# default is ucontext, but always fallback to gthread
-# windows autodetected by make
-if test "$coroutine" = "" -o "$coroutine" = "ucontext"; then
-  if test "$darwin" != "yes"; then
-    cat > $TMPC << EOF
+# We prefer ucontext, but it's not always possible. The fallback
+# is sigcontext. gthread is not selectable except explicitly, because
+# it is not functional enough to run QEMU proper. (It is occasionally
+# useful for debugging purposes.)  On Windows the only valid backend
+# is the Windows-specific one.
+
+ucontext_works=no
+if test "$darwin" != "yes"; then
+  cat > $TMPC << EOF
 #include <ucontext.h>
 #ifdef __stub_makecontext
 #error Ignoring glibc stub makecontext which will always fail
 #endif
 int main(void) { makecontext(0, 0, 0); return 0; }
 EOF
-    if compile_prog "" "" ; then
-        coroutine_backend=ucontext
-    else
-       coroutine_backend=gthread
-    fi
+  if compile_prog "" "" ; then
+    ucontext_works=yes
+  fi
+fi
+
+if test "$coroutine" = ""; then
+  if test "$mingw32" = "yes"; then
+    coroutine=windows
+  elif test "$ucontext_works" = "yes"; then
+    coroutine=ucontext
+  else
+    coroutine=sigaltstack
   fi
-elif test "$coroutine" = "gthread" ; then
-  coroutine_backend=gthread
-elif test "$coroutine" = "windows" ; then
-  coroutine_backend=windows
-elif test "$coroutine" = "sigaltstack" ; then
-  coroutine_backend=sigaltstack
 else
-  error_exit "unknown coroutine backend $coroutine"
+  case $coroutine in
+  windows)
+    if test "$mingw32" != "yes"; then
+      error_exit "'windows' coroutine backend only valid for Windows"
+    fi
+    ;;
+  ucontext)
+    if test "$ucontext_works" != "yes"; then
+      feature_not_found "ucontext"
+    fi
+    ;;
+  gthread|sigaltstack)
+    if test "$mingw32" = "yes"; then
+      error_exit "only the 'windows' coroutine backend is valid for Windows"
+    fi
+    ;;
+  *)
+    error_exit "unknown coroutine backend $coroutine"
+    ;;
+  esac
 fi
 
 ##########################################
@@ -3383,7 +3407,7 @@ echo "OpenGL support    $opengl"
 echo "libiscsi support  $libiscsi"
 echo "build guest agent $guest_agent"
 echo "seccomp support   $seccomp"
-echo "coroutine backend $coroutine_backend"
+echo "coroutine backend $coroutine"
 echo "GlusterFS support $glusterfs"
 echo "virtio-blk-data-plane $virtio_blk_data_plane"
 echo "gcov              $gcov_tool"
@@ -3713,11 +3737,8 @@ if test "$rbd" = "yes" ; then
   echo "CONFIG_RBD=y" >> $config_host_mak
 fi
 
-if test "$coroutine_backend" = "ucontext" ; then
-  echo "CONFIG_UCONTEXT_COROUTINE=y" >> $config_host_mak
-elif test "$coroutine_backend" = "sigaltstack" ; then
-  echo "CONFIG_SIGALTSTACK_COROUTINE=y" >> $config_host_mak
-fi
+def="CONFIG_$(upper $coroutine)_COROUTINE"
+echo "$def=y" >> $config_host_mak
 
 if test "$open_by_handle_at" = "yes" ; then
   echo "CONFIG_OPEN_BY_HANDLE=y" >> $config_host_mak
-- 
1.7.9.5