accel/tcg: Remove CF_LAST_IO

In cpu_exec_step_atomic, we did not set CF_LAST_IO, which lead
to a loop with cpu_io_recompile.

But since 18a536f1f8 ("Always require can_do_io") we no longer
need a flag to indicate when the last insn should have can_do_io set,
so remove the flag entirely.

Reported-by: Clément Chigot <chigot@adacore.com>
Tested-by: Clément Chigot <chigot@adacore.com>
Reviewed-by: Claudio Fontana <cfontana@suse.de>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1961
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

accel/tcg/cpu-exec.c

@@ -721,7 +721,7 @@ static inline bool cpu_handle_exception(CPUState *cpu, int *ret)
             && cpu->neg.icount_decr.u16.low + cpu->icount_extra == 0) {
             /* Execute just one insn to trigger exception pending in the log */
             cpu->cflags_next_tb = (curr_cflags(cpu) & ~CF_USE_ICOUNT)
-                | CF_LAST_IO | CF_NOIRQ | 1;
+                | CF_NOIRQ | 1;
         }
 #endif
         return false;

accel/tcg/tb-maint.c

@@ -1083,8 +1083,7 @@ bool tb_invalidate_phys_page_unwind(tb_page_addr_t addr, uintptr_t pc)
     if (current_tb_modified) {
         /* Force execution of one insn next time.  */
         CPUState *cpu = current_cpu;
-        cpu->cflags_next_tb =
-            1 | CF_LAST_IO | CF_NOIRQ | curr_cflags(current_cpu);
+        cpu->cflags_next_tb = 1 | CF_NOIRQ | curr_cflags(current_cpu);
         return true;
     }
     return false;
@@ -1154,8 +1153,7 @@ tb_invalidate_phys_page_range__locked(struct page_collection *pages,
     if (current_tb_modified) {
         page_collection_unlock(pages);
         /* Force execution of one insn next time.  */
-        current_cpu->cflags_next_tb =
-            1 | CF_LAST_IO | CF_NOIRQ | curr_cflags(current_cpu);
+        current_cpu->cflags_next_tb = 1 | CF_NOIRQ | curr_cflags(current_cpu);
         mmap_unlock();
         cpu_loop_exit_noexc(current_cpu);
     }

accel/tcg/translate-all.c

@@ -304,7 +304,7 @@ TranslationBlock *tb_gen_code(CPUState *cpu,
 
     if (phys_pc == -1) {
         /* Generate a one-shot TB with 1 insn in it */
-        cflags = (cflags & ~CF_COUNT_MASK) | CF_LAST_IO | 1;
+        cflags = (cflags & ~CF_COUNT_MASK) | 1;
     }
 
     max_insns = cflags & CF_COUNT_MASK;
@@ -632,7 +632,7 @@ void cpu_io_recompile(CPUState *cpu, uintptr_t retaddr)
      * operations only (which execute after completion) so we don't
      * double instrument the instruction.
      */
-    cpu->cflags_next_tb = curr_cflags(cpu) | CF_MEMI_ONLY | CF_LAST_IO | n;
+    cpu->cflags_next_tb = curr_cflags(cpu) | CF_MEMI_ONLY | n;
 
     if (qemu_loglevel_mask(CPU_LOG_EXEC)) {
         vaddr pc = log_pc(cpu, tb);

accel/tcg/translator.c

@@ -89,7 +89,7 @@ static TCGOp *gen_tb_start(DisasContextBase *db, uint32_t cflags)
      * 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 && (cflags & CF_LAST_IO));
+    set_can_do_io(db, db->max_insns == 1);
 
     return icount_start_insn;
 }
@@ -151,13 +151,7 @@ void translator_loop(CPUState *cpu, TranslationBlock *tb, int *max_insns,
     ops->tb_start(db, cpu);
     tcg_debug_assert(db->is_jmp == DISAS_NEXT);  /* no early exit */
 
-    if (cflags & CF_MEMI_ONLY) {
-        /* We should only see CF_MEMI_ONLY for io_recompile. */
-        assert(cflags & CF_LAST_IO);
-        plugin_enabled = plugin_gen_tb_start(cpu, db, true);
-    } else {
-        plugin_enabled = plugin_gen_tb_start(cpu, db, false);
-    }
+    plugin_enabled = plugin_gen_tb_start(cpu, db, cflags & CF_MEMI_ONLY);
     db->plugin_enabled = plugin_enabled;
 
     while (true) {
@@ -169,11 +163,13 @@ void translator_loop(CPUState *cpu, TranslationBlock *tb, int *max_insns,
             plugin_gen_insn_start(cpu, db);
         }
 
-        /* Disassemble one instruction.  The translate_insn hook should
-           update db->pc_next and db->is_jmp to indicate what should be
-           done next -- either exiting this loop or locate the start of
-           the next instruction.  */
-        if (db->num_insns == db->max_insns && (cflags & CF_LAST_IO)) {
+        /*
+         * Disassemble one instruction.  The translate_insn hook should
+         * update db->pc_next and db->is_jmp to indicate what should be
+         * 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);
         }

docs/devel/tcg-icount.rst

@@ -62,12 +62,6 @@ To deal with this case, when an I/O access is made we:
   - re-compile a single [1]_ instruction block for the current PC
   - exit the cpu loop and execute the re-compiled block
 
-The new block is created with the CF_LAST_IO compile flag which
-ensures the final instruction translation starts with a call to
-gen_io_start() so we don't enter a perpetual loop constantly
-recompiling a single instruction block. For translators using the
-common translator_loop this is done automatically.
-  
 .. [1] sometimes two instructions if dealing with delay slots  
 
 Other I/O operations

include/exec/translation-block.h

@@ -71,13 +71,12 @@ struct TranslationBlock {
 #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
-#define CF_INVALID       0x00040000 /* TB is stale. Set with @jmp_lock held */
-#define CF_PARALLEL      0x00080000 /* Generate code for a parallel context */
-#define CF_NOIRQ         0x00100000 /* Generate an uninterruptible TB */
-#define CF_PCREL         0x00200000 /* Opcodes in TB are PC-relative */
+#define CF_MEMI_ONLY     0x00001000 /* Only instrument memory ops */
+#define CF_USE_ICOUNT    0x00002000
+#define CF_INVALID       0x00004000 /* TB is stale. Set with @jmp_lock held */
+#define CF_PARALLEL      0x00008000 /* Generate code for a parallel context */
+#define CF_NOIRQ         0x00010000 /* Generate an uninterruptible TB */
+#define CF_PCREL         0x00020000 /* Opcodes in TB are PC-relative */
 #define CF_CLUSTER_MASK  0xff000000 /* Top 8 bits are cluster ID */
 #define CF_CLUSTER_SHIFT 24
 

system/watchpoint.c

@@ -179,8 +179,7 @@ void cpu_check_watchpoint(CPUState *cpu, vaddr addr, vaddr len,
                  */
                 if (!cpu->neg.can_do_io) {
                     /* Force execution of one insn next time.  */
-                    cpu->cflags_next_tb = 1 | CF_LAST_IO | CF_NOIRQ
-                                          | curr_cflags(cpu);
+                    cpu->cflags_next_tb = 1 | CF_NOIRQ | curr_cflags(cpu);
                     cpu_loop_exit_restore(cpu, ra);
                 }
                 /*
@@ -212,8 +211,7 @@ void cpu_check_watchpoint(CPUState *cpu, vaddr addr, vaddr len,
                 cpu_loop_exit(cpu);
             } else {
                 /* Force execution of one insn next time.  */
-                cpu->cflags_next_tb = 1 | CF_LAST_IO | CF_NOIRQ
-                                      | curr_cflags(cpu);
+                cpu->cflags_next_tb = 1 | CF_NOIRQ | curr_cflags(cpu);
                 mmap_unlock();
                 cpu_loop_exit_noexc(cpu);
             }