exec: drop cpu_can_do_io, just read cpu->can_do_io

After commit 626cf8f (icount: set can_do_io outside TB execution,
2014-12-08), can_do_io is set to 1 if not executing code.  It is
no longer necessary to make this assumption in cpu_can_do_io.

It is also possible to remove the use_icount test, simply by
never setting cpu->can_do_io to 0 unless use_icount is true.

With these changes cpu_can_do_io boils down to a read of
cpu->can_do_io.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

cpu-exec.c

@@ -196,7 +196,7 @@ static inline tcg_target_ulong cpu_tb_exec(CPUState *cpu, uint8_t *tb_ptr)
     }
 #endif /* DEBUG_DISAS */
 
-    cpu->can_do_io = 0;
+    cpu->can_do_io = !use_icount;
     next_tb = tcg_qemu_tb_exec(env, tb_ptr);
     cpu->can_do_io = 1;
     trace_exec_tb_exit((void *) (next_tb & ~TB_EXIT_MASK),

cpus.c

@@ -145,7 +145,7 @@ int64_t cpu_get_icount_raw(void)
 
     icount = timers_state.qemu_icount;
     if (cpu) {
-        if (!cpu_can_do_io(cpu)) {
+        if (!cpu->can_do_io) {
             fprintf(stderr, "Bad icount read\n");
             exit(1);
         }

include/exec/exec-all.h

@@ -346,27 +346,6 @@ extern int singlestep;
 /* cpu-exec.c */
 extern volatile sig_atomic_t exit_request;
 
-/**
- * cpu_can_do_io:
- * @cpu: The CPU for which to check IO.
- *
- * Deterministic execution requires that IO only be performed on the last
- * instruction of a TB so that interrupts take effect immediately.
- *
- * Returns: %true if memory-mapped IO is safe, %false otherwise.
- */
-static inline bool cpu_can_do_io(CPUState *cpu)
-{
-    if (!use_icount) {
-        return true;
-    }
-    /* If not executing code then assume we are ok.  */
-    if (cpu->current_tb == NULL) {
-        return true;
-    }
-    return cpu->can_do_io != 0;
-}
-
 #if !defined(CONFIG_USER_ONLY)
 void migration_bitmap_extend(ram_addr_t old, ram_addr_t new);
 #endif

include/qom/cpu.h

@@ -231,7 +231,9 @@ struct kvm_run;
  * @icount_decr: Number of cycles left, with interrupt flag in high bit.
  * This allows a single read-compare-cbranch-write sequence to test
  * for both decrementer underflow and exceptions.
- * @can_do_io: Nonzero if memory-mapped IO is safe.
+ * @can_do_io: Nonzero if memory-mapped IO is safe. Deterministic execution
+ * requires that IO only be performed on the last instruction of a TB
+ * so that interrupts take effect immediately.
  * @env_ptr: Pointer to subclass-specific CPUArchState field.
  * @current_tb: Currently executing TB.
  * @gdb_regs: Additional GDB registers.

qom/cpu.c

@@ -247,7 +247,7 @@ static void cpu_common_reset(CPUState *cpu)
     cpu->mem_io_vaddr = 0;
     cpu->icount_extra = 0;
     cpu->icount_decr.u32 = 0;
-    cpu->can_do_io = 0;
+    cpu->can_do_io = 1;
     cpu->exception_index = -1;
     memset(cpu->tb_jmp_cache, 0, TB_JMP_CACHE_SIZE * sizeof(void *));
 }

softmmu_template.h

@@ -154,7 +154,7 @@ static inline DATA_TYPE glue(io_read, SUFFIX)(CPUArchState *env,
 
     physaddr = (physaddr & TARGET_PAGE_MASK) + addr;
     cpu->mem_io_pc = retaddr;
-    if (mr != &io_mem_rom && mr != &io_mem_notdirty && !cpu_can_do_io(cpu)) {
+    if (mr != &io_mem_rom && mr != &io_mem_notdirty && !cpu->can_do_io) {
         cpu_io_recompile(cpu, retaddr);
     }
 
@@ -374,7 +374,7 @@ static inline void glue(io_write, SUFFIX)(CPUArchState *env,
     MemoryRegion *mr = iotlb_to_region(cpu, physaddr);
 
     physaddr = (physaddr & TARGET_PAGE_MASK) + addr;
-    if (mr != &io_mem_rom && mr != &io_mem_notdirty && !cpu_can_do_io(cpu)) {
+    if (mr != &io_mem_rom && mr != &io_mem_notdirty && !cpu->can_do_io) {
         cpu_io_recompile(cpu, retaddr);
     }
 

translate-all.c

@@ -222,6 +222,7 @@ static int cpu_restore_state_from_tb(CPUState *cpu, TranslationBlock *tb,
     gen_intermediate_code_pc(env, tb);
 
     if (tb->cflags & CF_USE_ICOUNT) {
+        assert(use_icount);
         /* Reset the cycle counter to the start of the block.  */
         cpu->icount_decr.u16.low += tb->icount;
         /* Clear the IO flag.  */
@@ -1470,7 +1471,7 @@ static void tcg_handle_interrupt(CPUState *cpu, int mask)
 
     if (use_icount) {
         cpu->icount_decr.u16.high = 0xffff;
-        if (!cpu_can_do_io(cpu)
+        if (!cpu->can_do_io
             && (mask & ~old_mask) != 0) {
             cpu_abort(cpu, "Raised interrupt while not in I/O function");
         }