tcg: enable MTTCG by default for ARM on x86 hosts

This enables the multi-threaded system emulation by default for ARMv7
and ARMv8 guests using the x86_64 TCG backend. This is because on the
guest side:

  - The ARM translate.c/translate-64.c have been converted to
    - use MTTCG safe atomic primitives
    - emit the appropriate barrier ops
  - The ARM machine has been updated to
    - hold the BQL when modifying shared cross-vCPU state
    - defer powerctl changes to async safe work

All the host backends support the barrier and atomic primitives but
need to provide same-or-better support for normal load/store
operations.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Acked-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Pranith Kumar <bobby.prani@gmail.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>

configure

@@ -5879,6 +5879,7 @@ mkdir -p $target_dir
 echo "# Automatically generated by configure - do not modify" > $config_target_mak
 
 bflt="no"
+mttcg="no"
 interp_prefix1=$(echo "$interp_prefix" | sed "s/%M/$target_name/g")
 gdb_xml_files=""
 
@@ -5897,11 +5898,13 @@ case "$target_name" in
   arm|armeb)
     TARGET_ARCH=arm
     bflt="yes"
+    mttcg="yes"
     gdb_xml_files="arm-core.xml arm-vfp.xml arm-vfp3.xml arm-neon.xml"
   ;;
   aarch64)
     TARGET_BASE_ARCH=arm
     bflt="yes"
+    mttcg="yes"
     gdb_xml_files="aarch64-core.xml aarch64-fpu.xml arm-core.xml arm-vfp.xml arm-vfp3.xml arm-neon.xml"
   ;;
   cris)
@@ -6066,6 +6069,9 @@ if test "$target_bigendian" = "yes" ; then
 fi
 if test "$target_softmmu" = "yes" ; then
   echo "CONFIG_SOFTMMU=y" >> $config_target_mak
+  if test "$mttcg" = "yes" ; then
+    echo "TARGET_SUPPORTS_MTTCG=y" >> $config_target_mak
+  fi
 fi
 if test "$target_user_only" = "yes" ; then
   echo "CONFIG_USER_ONLY=y" >> $config_target_mak

target/arm/cpu.h

@@ -30,6 +30,9 @@
 #  define TARGET_LONG_BITS 32
 #endif
 
+/* ARM processors have a weak memory model */
+#define TCG_GUEST_DEFAULT_MO      (0)
+
 #define CPUArchState struct CPUARMState
 
 #include "qemu-common.h"

tcg/i386/tcg-target.h

@@ -165,4 +165,15 @@ static inline void flush_icache_range(uintptr_t start, uintptr_t stop)
 {
 }
 
+/* This defines the natural memory order supported by this
+ * architecture before guarantees made by various barrier
+ * instructions.
+ *
+ * The x86 has a pretty strong memory ordering which only really
+ * allows for some stores to be re-ordered after loads.
+ */
+#include "tcg-mo.h"
+
+#define TCG_TARGET_DEFAULT_MO (TCG_MO_ALL & ~TCG_MO_ST_LD)
+
 #endif