Unify interrupt return paths from TSP into the TSPD

The TSP is expected to pass control back to EL3 if it gets preempted due to
an interrupt while handling a Standard SMC in the following scenarios:

1. An FIQ preempts Standard SMC execution and that FIQ is not a TSP Secure
   timer interrupt or is preempted by a higher priority interrupt by the time
   the TSP acknowledges it. In this case, the TSP issues an SMC with the ID
   as `TSP_EL3_FIQ`. Currently this case is never expected to happen as only
   the TSP Secure Timer is expected to generate FIQ.

2. An IRQ preempts Standard SMC execution and in this case the TSP issues
   an SMC with the ID as `TSP_PREEMPTED`.

In both the cases, the TSPD hands control back to the normal world and returns
returns an error code to the normal world to indicate that the standard SMC it
had issued has been preempted but not completed.

This patch unifies the handling of these two cases in the TSPD and ensures that
the TSP only uses TSP_PREEMPTED instead of separate SMC IDs. Also instead of 2
separate error codes, SMC_PREEMPTED and TSP_EL3_FIQ, only SMC_PREEMPTED is
returned as error code back to the normal world.

Background information: On a GICv3 system, when the secure world has affinity
routing enabled, in 2. an FIQ will preempt TSP execution instead of an IRQ. The
FIQ could be a result of a Group 0 or a Group 1 NS interrupt. In both case, the
TSPD passes control back to the normal world upon receipt of the TSP_PREEMPTED
SMC. A Group 0 interrupt will immediately preempt execution to EL3 where it
will be handled. This allows for unified interrupt handling in TSP for both
GICv3 and GICv2 systems.

Change-Id: I9895344db74b188021e3f6a694701ad272fb40d4

1 files changed, 16 insertions, 31 deletions

diff --git a/services/spd/tspd/tspd_main.c b/services/spd/tspd/tspd_main.c
index 62231601..0d6e0d22 100644
--- a/services/spd/tspd/tspd_main.c
+++ b/services/spd/tspd/tspd_main.c
@@ -72,9 +72,16 @@ DEFINE_SVC_UUID(tsp_uuid,
 
 int32_t tspd_init(void);
 
+/*
+ * This helper function handles Secure EL1 preemption. The preemption could be
+ * due Non Secure interrupts or EL3 interrupts. In both the cases we context
+ * switch to the normal world and in case of EL3 interrupts, it will again be
+ * routed to EL3 which will get handled at the exception vectors.
+ */
 uint64_t tspd_handle_sp_preemption(void *handle)
 {
 	cpu_context_t *ns_cpu_context;
+
 	assert(handle == cm_get_context(SECURE));
 	cm_el1_sysregs_context_save(SECURE);
 	/* Get a reference to the non-secure context */
@@ -88,8 +95,16 @@ uint64_t tspd_handle_sp_preemption(void *handle)
 	cm_el1_sysregs_context_restore(NON_SECURE);
 	cm_set_next_eret_context(NON_SECURE);
 
+	/*
+	 * We need to restore non secure context according to
+	 * the SEL1 context which got preempted and currently
+	 * TSP can only be preempted when a STD SMC is ongoing.
+	 * Return SMC_PREEMPTED in x0 and restore non secure
+	 * context.
+	 */
 	SMC_RET1(ns_cpu_context, SMC_PREEMPTED);
 }
+
 /*******************************************************************************
  * This function is the handler registered for S-EL1 interrupts by the TSPD. It
  * validates the interrupt and upon success arranges entry into the TSP at
@@ -356,35 +371,6 @@ uint64_t tspd_smc_handler(uint32_t smc_fid,
 
 		SMC_RET0((uint64_t) ns_cpu_context);
 
-
-	/*
-	 * This function ID is used only by the TSP to indicate that it was
-	 * interrupted due to a EL3 FIQ interrupt. Execution should resume
-	 * in the normal world.
-	 */
-	case TSP_EL3_FIQ:
-		if (ns)
-			SMC_RET1(handle, SMC_UNK);
-
-		assert(handle == cm_get_context(SECURE));
-
-		/* Assert that standard SMC execution has been preempted */
-		assert(get_std_smc_active_flag(tsp_ctx->state));
-
-		/* Save the secure system register state */
-		cm_el1_sysregs_context_save(SECURE);
-
-		/* Get a reference to the non-secure context */
-		ns_cpu_context = cm_get_context(NON_SECURE);
-		assert(ns_cpu_context);
-
-		/* Restore non-secure state */
-		cm_el1_sysregs_context_restore(NON_SECURE);
-		cm_set_next_eret_context(NON_SECURE);
-
-		SMC_RET1(ns_cpu_context, TSP_EL3_FIQ);
-
-
 	/*
 	 * This function ID is used only by the SP to indicate it has
 	 * finished initialising itself after a cold boot
@@ -438,8 +424,7 @@ uint64_t tspd_smc_handler(uint32_t smc_fid,
 				panic();
 
 			/*
-			 * Disable the interrupt NS locally since it will be enabled globally
-			 * within cm_init_my_context.
+			 * Disable the NS interrupt locally.
 			 */
 			disable_intr_rm_local(INTR_TYPE_NS, SECURE);
 #endif