diff options
authorPeter Maydell <peter.maydell@linaro.org>2018-07-20 15:56:47 +0100
committerPeter Maydell <peter.maydell@linaro.org>2018-08-02 13:19:35 +0100
commit2052c3917f5428ca51d8e50c245291b10d35c600 (patch)
parent7a1ef343a1808d36dc9f8cc415511a954bc6ce7e (diff)
target/arm: Implement tailchaining for M profile corestarget-arm.for-3.1
Tailchaining is an optimization in handling of exception return for M-profile cores: if we are about to pop the exception stack for an exception return, but there is a pending exception which is higher priority than the priority we are returning to, then instead of unstacking and then immediately taking the exception and stacking registers again, we can chain to the pending exception without unstacking and stacking. For v6M and v7M it is IMPDEF whether tailchaining happens for pending exceptions; for v8M this is architecturally required. Implement it in QEMU for all M-profile cores, since in practice v6M and v7M hardware implementations generally do have it. (We were already doing tailchaining for derived exceptions which happened during exception return, like the validity checks and stack access failures; these have always been required to be tailchained for all versions of the architecture.) Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Reviewed-by: Richard Henderson <richard.henderson@linaro.org> Message-id: 20180720145647.8810-5-peter.maydell@linaro.org
1 files changed, 16 insertions, 0 deletions
diff --git a/target/arm/helper.c b/target/arm/helper.c
index 559065131a..8b07bf214e 100644
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -7162,6 +7162,22 @@ static void do_v7m_exception_exit(ARMCPU *cpu)
+ /*
+ * Tailchaining: if there is currently a pending exception that
+ * is high enough priority to preempt execution at the level we're
+ * about to return to, then just directly take that exception now,
+ * avoiding an unstack-and-then-stack. Note that now we have
+ * deactivated the previous exception by calling armv7m_nvic_complete_irq()
+ * our current execution priority is already the execution priority we are
+ * returning to -- none of the state we would unstack or set based on
+ * the EXCRET value affects it.
+ */
+ if (armv7m_nvic_can_take_pending_exception(env->nvic)) {
+ qemu_log_mask(CPU_LOG_INT, "...tailchaining to pending exception\n");
+ v7m_exception_taken(cpu, excret, true, false);
+ return;
+ }
switch_v7m_security_state(env, return_to_secure);