accel/tcg: Record singlestep_enabled in tb->cflags

Set CF_SINGLE_STEP when single-stepping is enabled.
This avoids the need to flush all tb's when turning
single-stepping on or off.

Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

accel/tcg/cpu-exec.c

@@ -150,10 +150,15 @@ uint32_t curr_cflags(CPUState *cpu)
     uint32_t cflags = cpu->tcg_cflags;
 
     /*
+     * Record gdb single-step.  We should be exiting the TB by raising
+     * EXCP_DEBUG, but to simplify other tests, disable chaining too.
+     *
      * For singlestep and -d nochain, suppress goto_tb so that
      * we can log -d cpu,exec after every TB.
      */
-    if (singlestep) {
+    if (unlikely(cpu->singlestep_enabled)) {
+        cflags |= CF_NO_GOTO_TB | CF_NO_GOTO_PTR | CF_SINGLE_STEP | 1;
+    } else if (singlestep) {
         cflags |= CF_NO_GOTO_TB | 1;
     } else if (qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
         cflags |= CF_NO_GOTO_TB;

accel/tcg/translate-all.c

@@ -1432,10 +1432,6 @@ TranslationBlock *tb_gen_code(CPUState *cpu,
     }
     QEMU_BUILD_BUG_ON(CF_COUNT_MASK + 1 != TCG_MAX_INSNS);
 
-    if (cpu->singlestep_enabled) {
-        max_insns = 1;
-    }
-
  buffer_overflow:
     tb = tcg_tb_alloc(tcg_ctx);
     if (unlikely(!tb)) {

accel/tcg/translator.c

@@ -38,11 +38,6 @@ bool translator_use_goto_tb(DisasContextBase *db, target_ulong dest)
         return false;
     }
 
-    /* Suppress goto_tb in the case of single-steping.  */
-    if (db->singlestep_enabled) {
-        return false;
-    }
-
     /* Check for the dest on the same page as the start of the TB.  */
     return ((db->pc_first ^ dest) & TARGET_PAGE_MASK) == 0;
 }
@@ -60,7 +55,7 @@ void translator_loop(const TranslatorOps *ops, DisasContextBase *db,
     db->is_jmp = DISAS_NEXT;
     db->num_insns = 0;
     db->max_insns = max_insns;
-    db->singlestep_enabled = cpu->singlestep_enabled;
+    db->singlestep_enabled = cflags & CF_SINGLE_STEP;
 
     ops->init_disas_context(db, cpu);
     tcg_debug_assert(db->is_jmp == DISAS_NEXT);  /* no early exit */

cpu.c

@@ -326,10 +326,6 @@ void cpu_single_step(CPUState *cpu, int enabled)
         cpu->singlestep_enabled = enabled;
         if (kvm_enabled()) {
             kvm_update_guest_debug(cpu, 0);
-        } else {
-            /* must flush all the translated code to avoid inconsistencies */
-            /* XXX: only flush what is necessary */
-            tb_flush(cpu);
         }
         trace_breakpoint_singlestep(cpu->cpu_index, enabled);
     }

include/exec/exec-all.h

@@ -497,6 +497,7 @@ struct TranslationBlock {
 #define CF_COUNT_MASK    0x000001ff
 #define CF_NO_GOTO_TB    0x00000200 /* Do not chain with goto_tb */
 #define CF_NO_GOTO_PTR   0x00000400 /* Do not chain with goto_ptr */
+#define CF_SINGLE_STEP   0x00000800 /* gdbstub single-step in effect */
 #define CF_LAST_IO       0x00008000 /* Last insn may be an IO access.  */
 #define CF_MEMI_ONLY     0x00010000 /* Only instrument memory ops */
 #define CF_USE_ICOUNT    0x00020000