target-arm: Split DISAS_YIELD from DISAS_WFE

Currently we use DISAS_WFE for both WFE and YIELD instructions.
This is functionally correct because at the moment both of them
are implemented as "yield this CPU back to the top level loop so
another CPU has a chance to run". However it's rather confusing
that YIELD ends up calling HELPER(wfe), and if we ever want to
implement real behaviour for WFE and SEV it's likely to trip us up.

Split out the yield codepath to use DISAS_YIELD and a new
HELPER(yield) function, and have HELPER(wfe) call HELPER(yield).
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Message-id: 1435672316-3311-2-git-send-email-peter.maydell@linaro.org
Reviewed-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>

target-arm/helper.h

@@ -50,6 +50,7 @@ DEF_HELPER_2(exception_internal, void, env, i32)
 DEF_HELPER_4(exception_with_syndrome, void, env, i32, i32, i32)
 DEF_HELPER_1(wfi, void, env)
 DEF_HELPER_1(wfe, void, env)
+DEF_HELPER_1(yield, void, env)
 DEF_HELPER_1(pre_hvc, void, env)
 DEF_HELPER_2(pre_smc, void, env, i32)
 

target-arm/op_helper.c

@@ -323,13 +323,25 @@ void HELPER(wfi)(CPUARMState *env)
 
 void HELPER(wfe)(CPUARMState *env)
 {
-    CPUState *cs = CPU(arm_env_get_cpu(env));
-
-    /* Don't actually halt the CPU, just yield back to top
+    /* This is a hint instruction that is semantically different
+     * from YIELD even though we currently implement it identically.
+     * Don't actually halt the CPU, just yield back to top
      * level loop. This is not going into a "low power state"
      * (ie halting until some event occurs), so we never take
      * a configurable trap to a different exception level.
      */
+    HELPER(yield)(env);
+}
+
+void HELPER(yield)(CPUARMState *env)
+{
+    ARMCPU *cpu = arm_env_get_cpu(env);
+    CPUState *cs = CPU(cpu);
+
+    /* This is a non-trappable hint instruction that generally indicates
+     * that the guest is currently busy-looping. Yield control back to the
+     * top level loop so that a more deserving VCPU has a chance to run.
+     */
     cs->exception_index = EXCP_YIELD;
     cpu_loop_exit(cs);
 }

target-arm/translate-a64.c

@@ -1199,6 +1199,8 @@ static void handle_hint(DisasContext *s, uint32_t insn,
         s->is_jmp = DISAS_WFI;
         return;
     case 1: /* YIELD */
+        s->is_jmp = DISAS_YIELD;
+        return;
     case 2: /* WFE */
         s->is_jmp = DISAS_WFE;
         return;
@@ -11107,6 +11109,10 @@ void gen_intermediate_code_internal_a64(ARMCPU *cpu,
             gen_a64_set_pc_im(dc->pc);
             gen_helper_wfe(cpu_env);
             break;
+        case DISAS_YIELD:
+            gen_a64_set_pc_im(dc->pc);
+            gen_helper_yield(cpu_env);
+            break;
         case DISAS_WFI:
             /* This is a special case because we don't want to just halt the CPU
              * if trying to debug across a WFI.

target-arm/translate.h

@@ -103,6 +103,7 @@ static inline int default_exception_el(DisasContext *s)
 #define DISAS_WFE 7
 #define DISAS_HVC 8
 #define DISAS_SMC 9
+#define DISAS_YIELD 10
 
 #ifdef TARGET_AARCH64
 void a64_translate_init(void);