accel/tcg: Improve can_do_io management

We already attempted to set and clear can_do_io before the first
and last insns, but only used the initial value of max_insns and
the call to translator_io_start to find those insns.

Now that we track insn_start in DisasContextBase, and now that
we have emit_before_op, we can wait until we have finished
translation to identify the true first and last insns and emit
the sets of can_do_io at that time.

This fixes the case of a translation block which crossed a page
boundary, and for which the second page turned out to be mmio.
In this case we truncate the block, and the previous logic for
can_do_io could leave a block with a single insn with can_do_io
set to false, which would fail an assertion in cpu_io_recompile.

Reported-by: Jørgen Hansen <Jorgen.Hansen@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Jørgen Hansen <Jorgen.Hansen@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

accel/tcg/translator.c

@@ -18,20 +18,14 @@
 
 static void set_can_do_io(DisasContextBase *db, bool val)
 {
-    if (db->saved_can_do_io != val) {
-        db->saved_can_do_io = val;
-
     QEMU_BUILD_BUG_ON(sizeof_field(CPUState, neg.can_do_io) != 1);
     tcg_gen_st8_i32(tcg_constant_i32(val), tcg_env,
                     offsetof(ArchCPU, parent_obj.neg.can_do_io) -
                     offsetof(ArchCPU, env));
-    }
 }
 
 bool translator_io_start(DisasContextBase *db)
 {
-    set_can_do_io(db, true);
-
     /*
      * Ensure that this instruction will be the last in the TB.
      * The target may override this to something more forceful.
@@ -84,13 +78,6 @@ static TCGOp *gen_tb_start(DisasContextBase *db, uint32_t cflags)
                          - offsetof(ArchCPU, env));
     }
 
-    /*
-     * cpu->neg.can_do_io is set automatically here at the beginning of
-     * each translation block.  The cost is minimal, plus it would be
-     * very easy to forget doing it in the translator.
-     */
-    set_can_do_io(db, db->max_insns == 1);
-
     return icount_start_insn;
 }
 
@@ -129,6 +116,7 @@ void translator_loop(CPUState *cpu, TranslationBlock *tb, int *max_insns,
 {
     uint32_t cflags = tb_cflags(tb);
     TCGOp *icount_start_insn;
+    TCGOp *first_insn_start = NULL;
     bool plugin_enabled;
 
     /* Initialize DisasContext */
@@ -139,7 +127,6 @@ void translator_loop(CPUState *cpu, TranslationBlock *tb, int *max_insns,
     db->num_insns = 0;
     db->max_insns = *max_insns;
     db->singlestep_enabled = cflags & CF_SINGLE_STEP;
-    db->saved_can_do_io = -1;
     db->insn_start = NULL;
     db->host_addr[0] = host_pc;
     db->host_addr[1] = NULL;
@@ -159,6 +146,9 @@ void translator_loop(CPUState *cpu, TranslationBlock *tb, int *max_insns,
         *max_insns = ++db->num_insns;
         ops->insn_start(db, cpu);
         db->insn_start = tcg_last_op();
+        if (first_insn_start == NULL) {
+            first_insn_start = db->insn_start;
+        }
         tcg_debug_assert(db->is_jmp == DISAS_NEXT);  /* no early exit */
 
         if (plugin_enabled) {
@@ -171,10 +161,6 @@ void translator_loop(CPUState *cpu, TranslationBlock *tb, int *max_insns,
          * done next -- either exiting this loop or locate the start of
          * the next instruction.
          */
-        if (db->num_insns == db->max_insns) {
-            /* Accept I/O on the last instruction.  */
-            set_can_do_io(db, true);
-        }
         ops->translate_insn(db, cpu);
 
         /*
@@ -207,6 +193,21 @@ void translator_loop(CPUState *cpu, TranslationBlock *tb, int *max_insns,
     ops->tb_stop(db, cpu);
     gen_tb_end(tb, cflags, icount_start_insn, db->num_insns);
 
+    /*
+     * Manage can_do_io for the translation block: set to false before
+     * the first insn and set to true before the last insn.
+     */
+    if (db->num_insns == 1) {
+        tcg_debug_assert(first_insn_start == db->insn_start);
+    } else {
+        tcg_debug_assert(first_insn_start != db->insn_start);
+        tcg_ctx->emit_before_op = first_insn_start;
+        set_can_do_io(db, false);
+    }
+    tcg_ctx->emit_before_op = db->insn_start;
+    set_can_do_io(db, true);
+    tcg_ctx->emit_before_op = NULL;
+
     if (plugin_enabled) {
         plugin_gen_tb_end(cpu, db->num_insns);
     }

include/exec/translator.h

@@ -87,7 +87,6 @@ typedef struct DisasContextBase {
     int num_insns;
     int max_insns;
     bool singlestep_enabled;
-    int8_t saved_can_do_io;
     bool plugin_enabled;
     struct TCGOp *insn_start;
     void *host_addr[2];