[Qemu-devel] [PATCH] target-i386: fix cmpxchg instruction emulation

[Andreas Gustafsson <gson@gson.org>]

When the i386 cmpxchg instruction is executed with a memory operand
and the comparison result is "unequal", do the memory write before
changing the accumulator instead of the other way around, because
otherwise the new accumulator value will incorrectly be used in the
comparison when the instruction is restarted after a page fault.

This bug was originally reported on 2010-04-25 as
https://bugs.launchpad.net/qemu/+bug/569760 

Signed-off-by: Andreas Gustafsson <gson@gson.org>
---
--- translate.c.orig	2011-12-09 18:21:28.000000000 +0200
+++ translate.c	2011-12-09 18:21:24.000000000 +0200
@@ -4870,20 +4870,23 @@ static target_ulong disas_insn(DisasCont
             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;