[PATCH v2 10/25] x86: mp: Support APs waiting for instructions

[Simon Glass]

At present the APs (non-boot CPUs) are inited once and then parked ready
for the OS to use them. However in some cases we want to send new requests
through, such as to change MTRRs and keep them consistent across CPUs.

Change the last state of the flight plan to go into a wait loop, accepting
instructions from the main CPU.

Signed-off-by: Simon Glass <sjg at chromium.org>
---

Changes in v2:
- Add more comments

 arch/x86/cpu/mp_init.c    | 126 +++++++++++++++++++++++++++++++++++---
 arch/x86/include/asm/mp.h |  11 ++++
 2 files changed, 128 insertions(+), 9 deletions(-)

diff --git a/arch/x86/cpu/mp_init.c b/arch/x86/cpu/mp_init.c
index cd4cae559d..673fdc9628 100644
--- a/arch/x86/cpu/mp_init.c
+++ b/arch/x86/cpu/mp_init.c
@@ -43,13 +43,38 @@ struct mp_flight_plan {
 	struct mp_flight_record *records;
 };
 
+/**
+ * struct mp_callback - Callback information for APs
+ *
+ * @func: Function to run
+ * @arg: Argument to pass to the function
+ * @logical_cpu_number: Either a CPU number (i.e. dev->req_seq) or a special
+ *	value like MP_SELECT_BSP. It tells the AP whether it should process this
+ *	callback
+ */
+struct mp_callback {
+	/**
+	 * func() - Function to call on the AP
+	 *
+	 * @arg: Argument to pass
+	 */
+	void (*func)(void *arg);
+	void *arg;
+	int logical_cpu_number;
+};
+
 static struct mp_flight_plan mp_info;
 
-struct cpu_map {
-	struct udevice *dev;
-	int apic_id;
-	int err_code;
-};
+/*
+ * ap_callbacks - Callback mailbox array
+ *
+ * Array of callback, one entry for each available CPU, indexed by the CPU
+ * number, which is dev->req_seq. The entry for the main CPU is never used.
+ * When this is NULL, there is no pending work for the CPU to run. When
+ * non-NULL it points to the mp_callback structure. This is shared between all
+ * CPUs, so should only be written by the main CPU.
+ */
+static struct mp_callback **ap_callbacks;
 
 static inline void barrier_wait(atomic_t *b)
 {
@@ -147,11 +172,9 @@ static void ap_init(unsigned int cpu_index)
 	debug("AP: slot %d apic_id %x, dev %s\n", cpu_index, apic_id,
 	      dev ? dev->name : "(apic_id not found)");
 
-	/* Walk the flight plan */
+	/* Walk the flight plan, which never returns */
 	ap_do_flight_plan(dev);
-
-	/* Park the AP */
-	debug("parking\n");
+	debug("Unexpected return\n");
 done:
 	stop_this_cpu();
 }
@@ -455,6 +478,86 @@ static int get_bsp(struct udevice **devp, int *cpu_countp)
 	return dev->req_seq;
 }
 
+/**
+ * read_callback() - Read the pointer in a callback slot
+ *
+ * This is called by APs to read their callback slow to see if there is a
+ * pointer to new instructions
+ *
+ * @slot: Pointer to the AP's callback slot
+ * @return value of that pointer
+ */
+static struct mp_callback *read_callback(struct mp_callback **slot)
+{
+	struct mp_callback *ret;
+
+	asm volatile ("mov	%1, %0\n"
+		: "=r" (ret)
+		: "m" (*slot)
+		: "memory"
+	);
+	return ret;
+}
+
+/**
+ * store_callback() - Store a pointer to the callback slot
+ *
+ * This is called by APs to write NULL into the callback slot when they have
+ * finished the work requested by the BSP.
+ *
+ * @slot: Pointer to the AP's callback slot
+ * @val: Value to write (e.g. NULL)
+ */
+static void store_callback(struct mp_callback **slot, struct mp_callback *val)
+{
+	asm volatile ("mov	%1, %0\n"
+		: "=m" (*slot)
+		: "r" (val)
+		: "memory"
+	);
+}
+
+/**
+ * ap_wait_for_instruction() - Wait for and process requests from the main CPU
+ *
+ * This is called by APs (here, everything other than the main boot CPU) to
+ * await instructions. They arrive in the form of a function call and argument,
+ * which is then called. This uses a simple mailbox with atomic read/set
+ *
+ * @cpu: CPU that is waiting
+ * @unused: Optional argument provided by struct mp_flight_record, not used here
+ * @return Does not return
+ */
+static int ap_wait_for_instruction(struct udevice *cpu, void *unused)
+{
+	struct mp_callback lcb;
+	struct mp_callback **per_cpu_slot;
+
+	per_cpu_slot = &ap_callbacks[cpu->req_seq];
+
+	while (1) {
+		struct mp_callback *cb = read_callback(per_cpu_slot);
+
+		if (!cb) {
+			asm ("pause");
+			continue;
+		}
+
+		/* Copy to local variable before using the value */
+		memcpy(&lcb, cb, sizeof(lcb));
+		mfence();
+		if (lcb.logical_cpu_number == MP_SELECT_ALL ||
+		    lcb.logical_cpu_number == MP_SELECT_APS ||
+		    cpu->req_seq == lcb.logical_cpu_number)
+			lcb.func(lcb.arg);
+
+		/* Indicate we are finished */
+		store_callback(per_cpu_slot, NULL);
+	}
+
+	return 0;
+}
+
 static int mp_init_cpu(struct udevice *cpu, void *unused)
 {
 	struct cpu_platdata *plat = dev_get_parent_platdata(cpu);
@@ -467,6 +570,7 @@ static int mp_init_cpu(struct udevice *cpu, void *unused)
 
 static struct mp_flight_record mp_steps[] = {
 	MP_FR_BLOCK_APS(mp_init_cpu, NULL, mp_init_cpu, NULL),
+	MP_FR_BLOCK_APS(ap_wait_for_instruction, NULL, NULL, NULL),
 };
 
 int mp_init(void)
@@ -504,6 +608,10 @@ int mp_init(void)
 	if (ret)
 		log_warning("Warning: Device tree does not describe all CPUs. Extra ones will not be started correctly\n");
 
+	ap_callbacks = calloc(num_cpus, sizeof(struct mp_callback *));
+	if (!ap_callbacks)
+		return -ENOMEM;
+
 	/* Copy needed parameters so that APs have a reference to the plan */
 	mp_info.num_records = ARRAY_SIZE(mp_steps);
 	mp_info.records = mp_steps;
diff --git a/arch/x86/include/asm/mp.h b/arch/x86/include/asm/mp.h
index 94af819ad9..41b1575f4b 100644
--- a/arch/x86/include/asm/mp.h
+++ b/arch/x86/include/asm/mp.h
@@ -11,6 +11,17 @@
 #include <asm/atomic.h>
 #include <asm/cache.h>
 
+enum {
+	/* Indicates that the function should run on all CPUs */
+	MP_SELECT_ALL	= -1,
+
+	/* Run on boot CPUs */
+	MP_SELECT_BSP	= -2,
+
+	/* Run on non-boot CPUs */
+	MP_SELECT_APS	= -3,
+};
+
 typedef int (*mp_callback_t)(struct udevice *cpu, void *arg);
 
 /*
-- 
2.27.0.290.gba653c62da-goog