[Qemu-devel] [PATCH] Improve cmpxchg emulation.

[Jonathan A. Kollasch]

Change the accumulator only after performing the redundant write during
cmpxchg.  This fixes pthreaded programs using fork() in NetBSD/i386
guests.

>From Andreas Gustafsson in https://bugs.launchpad.net/qemu/+bug/569760.

Signed-off-by: Jonathan A. Kollasch <address@hidden>
---
 target-i386/translate.c |   12 ++++++++----
 1 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/target-i386/translate.c b/target-i386/translate.c
index 7b6e3c2..391830f 100644
--- a/target-i386/translate.c
+++ b/target-i386/translate.c
@@ -4879,20 +4879,24 @@ static target_ulong disas_insn(DisasContext *s, 
target_ulong pc_start)
             tcg_gen_sub_tl(t2, cpu_regs[R_EAX], t0);
             gen_extu(ot, t2);
             tcg_gen_brcondi_tl(TCG_COND_EQ, t2, 0, label1);
+           label2 = gen_new_label();
             if (mod == 3) {
-                label2 = gen_new_label();
                 gen_op_mov_reg_v(ot, R_EAX, t0);
                 tcg_gen_br(label2);
                 gen_set_label(label1);
                 gen_op_mov_reg_v(ot, rm, t1);
-                gen_set_label(label2);
             } else {
-                tcg_gen_mov_tl(t1, t0);
+                /* perform no-op store cycle like physical cpu; must be
+                  before changing accumulator to ensure idempotency if
+                  the store faults and the instruction is restarted
+               */
+                gen_op_st_v(ot + s->mem_index, t0, a0);
                 gen_op_mov_reg_v(ot, R_EAX, t0);
+                tcg_gen_br(label2);
                 gen_set_label(label1);
-                /* always store */
                 gen_op_st_v(ot + s->mem_index, t1, a0);
             }
+           gen_set_label(label2);
             tcg_gen_mov_tl(cpu_cc_src, t0);
             tcg_gen_mov_tl(cpu_cc_dst, t2);
             s->cc_op = CC_OP_SUBB + ot;
-- 
1.6.6.2