[Qemu-devel] [PATCH] target-i386: Fix eflags.TF/#DB handling of syscall/sysret insns

[Doug Evans]

Hi.

While qemu's behaviour matches what one would expect from reading
the docs, it does not match what I'm seeing on h/w.
Can anyone else confirm what the correct behaviour is here?

---

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.

Signed-off-by: Doug Evans <address@hidden>
---
 target-i386/bpt_helper.c | 12 ++++++++++++
 target-i386/helper.h     |  1 +
 target-i386/translate.c  | 29 ++++++++++++++++++++++++-----
 3 files changed, 37 insertions(+), 5 deletions(-)

diff --git a/target-i386/bpt_helper.c b/target-i386/bpt_helper.c
index 6fd7fe0..d771461 100644
--- a/target-i386/bpt_helper.c
+++ b/target-i386/bpt_helper.c
@@ -244,6 +244,18 @@ void helper_single_step(CPUX86State *env)
     raise_exception(env, EXCP01_DB);
 }

+void helper_rechecking_single_step(CPUX86State *env)
+{
+    if ((env->eflags & TF_MASK) != 0)
+    {
+#ifndef CONFIG_USER_ONLY
+        check_hw_breakpoints(env, true);
+        env->dr[6] |= DR6_BS;
+#endif
+        raise_exception(env, EXCP01_DB);
+    }
+}
+
 void helper_set_dr(CPUX86State *env, int reg, target_ulong t0)
 {
 #ifndef CONFIG_USER_ONLY
diff --git a/target-i386/helper.h b/target-i386/helper.h
index 4e859eb..bd9b2cf 100644
--- a/target-i386/helper.h
+++ b/target-i386/helper.h
@@ -79,6 +79,7 @@ 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)
diff --git a/target-i386/translate.c b/target-i386/translate.c
index 9fd1a04..42d036e 100644
--- a/target-i386/translate.c
+++ b/target-i386/translate.c
@@ -2500,8 +2500,10 @@ 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.  */
-static void gen_eob_inhibit_irq(DisasContext *s, bool inhibit)
+   If INHIBIT, set HF_INHIBIT_IRQ_MASK if it isn't already set.
+   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)
 {
     gen_update_cc_op(s);

@@ -2517,6 +2519,9 @@ static void gen_eob_inhibit_irq(DisasContext *s, bool  
inhibit)
     }
     if (s->singlestep_enabled) {
         gen_helper_debug(cpu_env);
+    } else if (recheck_tf) {
+        gen_helper_rechecking_single_step(cpu_env);
+        tcg_gen_exit_tb(0);
     } else if (s->tf) {
         gen_helper_single_step(cpu_env);
     } else {
@@ -2525,10 +2530,17 @@ static void gen_eob_inhibit_irq(DisasContext *s,  
bool inhibit)
     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
@@ -7108,7 +7120,10 @@ 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 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 0x107: /* sysret */
         if (!s->pe) {
@@ -7119,7 +7134,11 @@ 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
--
2.8.0.rc3.226.g39d4020