[RFC PATCH for 5.0] configure: disable MTTCG for MIPS guests

[Alex Bennée]

While debugging check-acceptance failures I found an instability in
the mips64el test case. Briefly the test case:

  retry.py -n 100 -c -- ./mips64el-softmmu/qemu-system-mips64el \
    -display none -vga none -serial mon:stdio \
    -machine malta -kernel ./vmlinux-4.7.0-rc1.I6400 \
    -cpu I6400 -smp 8 -vga std \
    -append "printk.time=0 clocksource=GIC console=tty0 console=ttyS0 panic=-1" 
\
    --no-reboot

Reports about a 9% failure rate:

  Results summary:
  0: 91 times (91.00%), avg time 5.547 (0.45 varience/0.67 deviation)
  -6: 9 times (9.00%), avg time 3.394 (0.02 varience/0.13 deviation)
  Ran command 100 times, 91 passes

When re-run with "--accel tcg,thread=single" the instability goes
away.

  Results summary:
  0: 100 times (100.00%), avg time 17.318 (249.76 varience/15.80 deviation)
  Ran command 100 times, 100 passes

Which seems to indicate there is some aspect of the MIPS MTTCG fixes
that has been missed. Ideally we would fix that but I'm afraid I don't
have time to investigate and am not super familiar with the
architecture anyway.

I've disabled all the mips guests as I assume it's a fundamental
synchronisation primitive that is broken but I haven't tested them all
(there are a lot!).

Signed-off-by: Alex Bennée <address@hidden>
Cc: Aleksandar Markovic <address@hidden>
Cc: Aurelien Jarno <address@hidden>
Cc: Aleksandar Rikalo <address@hidden>
Cc: Philippe Mathieu-Daudé <address@hidden>
---
 configure | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/configure b/configure
index 206d22c5153..002792d21dc 100755
--- a/configure
+++ b/configure
@@ -7832,19 +7832,19 @@ case "$target_name" in
     echo "TARGET_ABI32=y" >> $config_target_mak
   ;;
   mips|mipsel)
-    mttcg="yes"
+    mttcg="no"
     TARGET_ARCH=mips
     echo "TARGET_ABI_MIPSO32=y" >> $config_target_mak
   ;;
   mipsn32|mipsn32el)
-    mttcg="yes"
+    mttcg="no"
     TARGET_ARCH=mips64
     TARGET_BASE_ARCH=mips
     echo "TARGET_ABI_MIPSN32=y" >> $config_target_mak
     echo "TARGET_ABI32=y" >> $config_target_mak
   ;;
   mips64|mips64el)
-    mttcg="yes"
+    mttcg="no"
     TARGET_ARCH=mips64
     TARGET_BASE_ARCH=mips
     echo "TARGET_ABI_MIPSN64=y" >> $config_target_mak
-- 
2.20.1