target-i386: Fix eflags.TF/#DB handling of syscall/sysret insns

The syscall and sysret instructions behave a bit differently:
TF is checked after the instruction completes.
This allows the o/s to disable #DB at a syscall by adding TF to FMASK.
And then when the sysret is executed the #DB is taken "as if" the
syscall insn just completed.

Backports commit c52ab08aee6f7d4717fc6b517174043126bd302f from qemu

qemu/target-i386/bpt_helper.c

@@ -243,6 +243,13 @@ void helper_single_step(CPUX86State *env)
     raise_exception(env, EXCP01_DB);
 }
 
+void helper_rechecking_single_step(CPUX86State *env)
+{
+    if ((env->eflags & TF_MASK) != 0) {
+        helper_single_step(env);
+    }
+}
+
 void helper_set_dr(CPUX86State *env, int reg, target_ulong t0)
 {
 #ifndef CONFIG_USER_ONLY

qemu/target-i386/helper.h

@@ -81,6 +81,7 @@ DEF_HELPER_2(cmpxchg16b_unlocked, void, env, tl)
 DEF_HELPER_2(cmpxchg16b, void, env, tl)
 #endif
 DEF_HELPER_1(single_step, void, env)
+DEF_HELPER_1(rechecking_single_step, void, env)
 DEF_HELPER_1(cpuid, void, env)
 DEF_HELPER_1(rdtsc, void, env)
 DEF_HELPER_1(rdtscp, void, env)

qemu/target-i386/translate.c

@@ -2837,8 +2837,10 @@ static void gen_bnd_jmp(DisasContext *s)
 }
 
 /* Generate an end of block. Trace exception is also generated if needed.
-   If IIM, set HF_INHIBIT_IRQ_MASK if it isn't already set.  */
+   If INHIBIT, set HF_INHIBIT_IRQ_MASK if it isn't already set.
-static void gen_eob_inhibit_irq(DisasContext *s, bool inhibit)
+   If RECHECK_TF, emit a rechecking helper for #DB, ignoring the state of
+   S->TF.  This is used by the syscall/sysret insns.  */
+static void gen_eob_worker(DisasContext *s, bool inhibit, bool recheck_tf)
 {
     TCGContext *tcg_ctx = s->uc->tcg_ctx;
 
@@ -2856,18 +2858,28 @@ static void gen_eob_inhibit_irq(DisasContext *s, bool inhibit)
     }
     if (s->singlestep_enabled) {
         gen_helper_debug(tcg_ctx, tcg_ctx->cpu_env);
+    } else if (recheck_tf) {
+        gen_helper_rechecking_single_step(tcg_ctx, tcg_ctx->cpu_env);
+        tcg_gen_exit_tb(tcg_ctx, 0);
     } else if (s->tf) {
         gen_helper_single_step(tcg_ctx, tcg_ctx->cpu_env);
     } else {
-        tcg_gen_exit_tb(s->uc->tcg_ctx, 0);
+        tcg_gen_exit_tb(tcg_ctx, 0);
     }
     s->is_jmp = DISAS_TB_JUMP;
 }
 
+/* End of block.
+   If INHIBIT, set HF_INHIBIT_IRQ_MASK if it isn't already set.  */
+static void gen_eob_inhibit_irq(DisasContext *s, bool inhibit)
+{
+    gen_eob_worker(s, inhibit, false);
+}
+
 /* End of block, resetting the inhibit irq flag.  */
 static void gen_eob(DisasContext *s)
 {
-    gen_eob_inhibit_irq(s, false);
+    gen_eob_worker(s, false, false);
 }
 
 /* generate a jump to eip. No segment change must happen before as a
@@ -7089,7 +7101,10 @@ static target_ulong disas_insn(CPUX86State *env, DisasContext *s,
                                       tcg_const_i32(tcg_ctx, s->pc - s->cs_base));
             set_cc_op(s, CC_OP_EFLAGS);
         }
-        gen_eob(s);
+        /* TF handling for the syscall insn is different. The TF bit is checked
+           after the syscall insn completes. This allows #DB to not be
+           generated after one has entered CPL0 if TF is set in FMASK.  */
+        gen_eob_worker(s, false, true);
         break;
     case 0xe8: /* call im */
         {
@@ -7797,7 +7812,11 @@ static target_ulong disas_insn(CPUX86State *env, DisasContext *s,
             if (s->lma) {
                 set_cc_op(s, CC_OP_EFLAGS);
             }
-            gen_eob(s);
+            /* TF handling for the sysret insn is different. The TF bit is
+               checked after the sysret insn completes. This allows #DB to be
+               generated "as if" the syscall insn in userspace has just
+               completed.  */
+            gen_eob_worker(s, false, true);
         }
         break;
 #endif