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: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1435672316-3311-2-git-send-email-peter.maydell@linaro.org
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>

1 files changed, 15 insertions, 3 deletions

diff --git a/target-arm/op_helper.c b/target-arm/op_helper.c
index 7fa32c4707..663c05d1d2 100644
--- a/target-arm/op_helper.c
+++ b/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);
 }