target/arm: Remove unnecessary gen_io_end() calls

Since commit ba3e792669 it has been unnecessary for target code
to call gen_io_end() after an IO instruction in icount mode; it is
sufficient to call gen_io_start() before it and to force the end of
the TB.

Many now-unnecessary calls to gen_io_end() were removed in commit
9e9b10c649, but some were missed or accidentally added later.
Remove unneeded calls from the arm target:

 * the call in the handling of exception-return-via-LDM is
   unnecessary, and the code is already forcing end-of-TB
 * the call in the VFP access check code is more complicated:
   we weren't ending the TB, so we need to add the code to
   force that by setting DISAS_UPDATE
 * the doc comment for ARM_CP_IO doesn't need to mention
   gen_io_end() any more

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>
Message-id: 20200619170324.12093-1-peter.maydell@linaro.org

target/arm/cpu.h

@@ -2334,7 +2334,7 @@ static inline uint64_t cpreg_to_kvm_id(uint32_t cpregid)
  * migration or KVM state synchronization. (Typically this is for "registers"
  * which are actually used as instructions for cache maintenance and so on.)
  * IO indicates that this register does I/O and therefore its accesses
- * need to be surrounded by gen_io_start()/gen_io_end(). In particular,
+ * need to be marked with gen_io_start() and also end the TB. In particular,
  * registers which implement clocks or timers require this.
  * RAISES_EXC is for when the read or write hook might raise an exception;
  * the generated code will synchronize the CPU state before calling the hook

target/arm/translate-vfp.inc.c

@@ -119,15 +119,14 @@ static bool full_vfp_access_check(DisasContext *s, bool ignore_vfp_enabled)
         if (s->v7m_lspact) {
             /*
              * Lazy state saving affects external memory and also the NVIC,
-             * so we must mark it as an IO operation for icount.
+             * so we must mark it as an IO operation for icount (and cause
+             * this to be the last insn in the TB).
              */
             if (tb_cflags(s->base.tb) & CF_USE_ICOUNT) {
+                s->base.is_jmp = DISAS_UPDATE;
                 gen_io_start();
             }
             gen_helper_v7m_preserve_fp_state(cpu_env);
-            if (tb_cflags(s->base.tb) & CF_USE_ICOUNT) {
-                gen_io_end();
-            }
             /*
              * If the preserve_fp_state helper doesn't throw an exception
              * then it will clear LSPACT; we don't need to repeat this for

target/arm/translate.c

@@ -7785,9 +7785,6 @@ static bool do_ldm(DisasContext *s, arg_ldst_block *a, int min_n)
             gen_io_start();
         }
         gen_helper_cpsr_write_eret(cpu_env, tmp);
-        if (tb_cflags(s->base.tb) & CF_USE_ICOUNT) {
-            gen_io_end();
-        }
         tcg_temp_free_i32(tmp);
         /* Must exit loop to check un-masked IRQs */
         s->base.is_jmp = DISAS_EXIT;