Add BASEPRI zero-latency ISR support

include/platform/irq-latency.h

@@ -0,0 +1,125 @@
+/* Copyright (c) 2026 The F9 Microkernel Project. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef PLATFORM_IRQ_LATENCY_H_
+#define PLATFORM_IRQ_LATENCY_H_
+
+#include <stdint.h>
+
+/* DWT (Data Watchpoint and Trace) registers for cycle counting */
+#define DWT_CTRL ((volatile uint32_t *) 0xE0001000)
+#define DWT_CYCCNT ((volatile uint32_t *) 0xE0001004)
+#define DWT_CTRL_CYCCNTENA (1 << 0)
+
+#define DEMCR ((volatile uint32_t *) 0xE000EDFC)
+#define DEMCR_TRCENA (1 << 24)
+
+/**
+ * @file irq_latency.h
+ * @brief Interrupt latency measurement and profiling infrastructure
+ *
+ * Provides cycle-accurate latency tracking for zero-latency ISRs and
+ * standard IRQs. Enables validation of BASEPRI-based zero-latency
+ * interrupt performance (<10 cycle target).
+ *
+ * Usage:
+ *   1. Call latency_sample_start() at ISR entry
+ *   2. Call latency_sample_end(priority, irq_num) at ISR exit
+ *   3. View statistics via KDB 'L' command
+ */
+
+/**
+ * Latency statistics per interrupt priority level.
+ */
+typedef struct {
+    uint32_t count; /* Number of samples */
+    uint32_t min;   /* Minimum latency (cycles) */
+    uint32_t max;   /* Maximum latency (cycles) */
+    uint32_t sum;   /* Sum for average calculation */
+    uint32_t avg;   /* Average latency (cycles) */
+} latency_stats_t;
+
+/**
+ * Get current cycle count from DWT_CYCCNT.
+ * Returns 0 if DWT is not enabled.
+ */
+static inline uint32_t get_cycle_count(void)
+{
+    return *DWT_CYCCNT;
+}
+
+/**
+ * Enable DWT cycle counter for latency measurements.
+ * Called during system initialization.
+ */
+void latency_init(void);
+
+/**
+ * Record latency sample for an interrupt.
+ *
+ * @param priority Interrupt priority (0x0-0xF)
+ * @param irq_num IRQ number (-15 to 239)
+ * @param cycles Measured latency in cycles
+ */
+void latency_record(uint8_t priority, int16_t irq_num, uint32_t cycles);
+
+/**
+ * Get latency statistics for a priority level.
+ *
+ * @param priority Interrupt priority (0x0-0xF)
+ * @return Pointer to statistics structure
+ */
+const latency_stats_t *latency_get_stats(uint8_t priority);
+
+/**
+ * Get a best-effort atomic snapshot of latency statistics.
+ *
+ * Uses relaxed atomics only; intended for diagnostic reads outside ISR
+ * context. Returns 1 on success, 0 on invalid input.
+ */
+int latency_get_stats_snapshot(uint8_t priority, latency_stats_t *out);
+
+/**
+ * Reset all latency statistics.
+ */
+void latency_reset(void);
+
+/**
+ * Get interrupt number from IPSR.
+ * Returns 0 for thread mode, 1-15 for exceptions, 16+ for IRQs.
+ */
+static inline uint32_t get_irq_number(void)
+{
+    uint32_t ipsr;
+    __asm__ __volatile__("mrs %0, ipsr" : "=r"(ipsr));
+    return ipsr & 0x1FF;
+}
+
+/**
+ * Latency measurement helper - call at ISR entry.
+ * Returns timestamp for latency_sample_end().
+ */
+static inline uint32_t latency_sample_start(void)
+{
+    return get_cycle_count();
+}
+
+/**
+ * Latency measurement helper - call at ISR exit.
+ *
+ * @param start_cycles Timestamp from latency_sample_start()
+ * @param priority Interrupt priority level
+ * @param irq_num IRQ number from IPSR
+ */
+static inline void latency_sample_end(uint32_t start_cycles,
+                                      uint8_t priority,
+                                      int16_t irq_num)
+{
+    uint32_t end_cycles = get_cycle_count();
+    uint32_t elapsed = end_cycles - start_cycles;
+    latency_record(priority, irq_num, elapsed);
+}
+
+#endif /* PLATFORM_IRQ_LATENCY_H_ */

include/platform/irq.h

@@ -15,6 +15,34 @@
 
 void irq_init(void);
 
+/*
+ * Interrupt Priority Levels (ARM Cortex-M 4-bit priorities)
+ */
+#define IRQ_PRIO_ZERO_LATENCY_MAX 0x2 /* Highest priority, never masked */
+#define IRQ_PRIO_SYSTICK 0x3          /* System timer */
+#define IRQ_PRIO_KERNEL_MASK 0x40     /* BASEPRI mask (0x4 << 4) */
+#define IRQ_PRIO_USER_DEFAULT 0x8     /* Default user IRQ priority */
+#define IRQ_PRIO_LOWEST 0xF           /* SVCall, PendSV */
+
+/*
+ * System state tracking for ISR context.
+ * 0 = Thread mode (PSP), 1+ = Handler mode (MSP, tracks nesting depth).
+ */
+extern volatile uint32_t irq_system_state;
+
+/*
+ * Fast ISR context check using hardware IPSR register.
+ * Returns: true if currently in exception handler, false if in thread mode.
+ * Zero overhead: Single MRS instruction, no memory access or race conditions.
+ */
+static inline bool in_isr_context(void)
+{
+    return IPSR() != 0;
+}
+
+/*
+ * PRIMASK-based critical sections (blocks ALL interrupts).
+ */
 static inline void irq_disable(void)
 {
     __asm__ __volatile__("cpsid i" ::: "memory");
@@ -45,6 +73,53 @@ static inline void irq_restore_flags(uint32_t flags)
     __asm__ __volatile__("msr primask, %0" ::"r"(flags) : "memory");
 }
 
+/*
+ * BASEPRI-based critical sections (blocks interrupts >= priority level).
+ * Zero-latency ISRs at priority 0x0-0x2 can preempt kernel critical sections.
+ */
+static inline void irq_disable_below(uint8_t priority)
+{
+    uint32_t basepri = (priority << 4) & 0xFF;
+    __asm__ __volatile__("msr basepri, %0" ::"r"(basepri) : "memory");
+}
+
+static inline void irq_enable_all(void)
+{
+    __asm__ __volatile__("msr basepri, %0" ::"r"(0) : "memory");
+}
+
+static inline uint32_t irq_save_basepri(uint8_t priority)
+{
+    uint32_t prev_basepri;
+    uint32_t new_basepri = (priority << 4) & 0xFF;
+    __asm__ __volatile__(
+        "mrs %0, basepri\n\t"
+        "msr basepri, %1"
+        : "=r"(prev_basepri)
+        : "r"(new_basepri)
+        : "memory");
+    return prev_basepri;
+}
+
+static inline void irq_restore_basepri(uint32_t basepri)
+{
+    __asm__ __volatile__("msr basepri, %0" ::"r"(basepri) : "memory");
+}
+
+/*
+ * Kernel critical section (masks interrupts >= 0x4, allows 0x0-0x3).
+ * Use this as the default for scheduler, IPC, and memory operations.
+ */
+static inline uint32_t irq_kernel_critical_enter(void)
+{
+    return irq_save_basepri(IRQ_PRIO_KERNEL_MASK >> 4);
+}
+
+static inline void irq_kernel_critical_exit(uint32_t basepri)
+{
+    irq_restore_basepri(basepri);
+}
+
 static inline void irq_svc(void)
 {
     __asm__ __volatile__("svc #0");
@@ -242,7 +317,6 @@ extern volatile uint32_t __irq_saved_regs[8];
         request_schedule();    \
         irq_return();          \
     }
-
 extern volatile tcb_t *current;
 
 #endif /* PLATFORM_IRQ_H_ */

kernel/build.mk

@@ -21,7 +21,8 @@ kernel-y = \
 	interrupt.o
 
 KDB-$(CONFIG_KDB) = \
-	kdb.o
+	kdb.o \
+	kdb-latency.o
 
 KPROBES-$(CONFIG_KPROBES) = \
 	kprobes.o

kernel/ipc.c

@@ -214,18 +214,19 @@ static void do_ipc(tcb_t *from, tcb_t *to)
      * CONSTRAINT: Callback MUST NOT destroy its own TCB.
      */
     if (to->ipc_notify && to->notify_pending && to->notify_depth < 3) {
-        uint32_t irq_flags;
+        uint32_t basepri;
         uint8_t generation_before;
         notify_handler_t callback;
 
         /* Atomically increment depth and capture generation.
-         * IRQ masking prevents race with nested interrupt-driven IPC.
+         * BASEPRI masking prevents race with nested interrupt-driven IPC.
+         * Zero-latency ISRs (0x0-0x2) can still preempt during this operation.
          */
-        irq_flags = irq_save_flags();
+        basepri = irq_kernel_critical_enter();
         to->notify_depth++;
         generation_before = to->notify_generation;
         callback = to->ipc_notify;
-        irq_restore_flags(irq_flags);
+        irq_kernel_critical_exit(basepri);
 
         /* Recursion protection: prevent unbounded callback nesting.
          * Max depth 3 allows: serial → network → timer notification chains.
@@ -245,11 +246,27 @@ static void do_ipc(tcb_t *from, tcb_t *to)
         /* Atomically decrement depth only if TCB still valid.
          * Generation counter detects TCB destruction during callback.
          * If TCB was destroyed, skip depth decrement (would be use-after-free).
+         *
+         * SAFETY: We must verify 'to' is still a valid TCB before accessing it.
+         * Search thread_map to confirm the pointer hasn't been freed and
+         * reused.
          */
-        irq_flags = irq_save_flags();
-        if (to->notify_generation == generation_before)
+        basepri = irq_kernel_critical_enter();
+
+        /* Verify TCB is still valid by checking thread_map */
+        int tcb_valid = 0;
+        for (int i = 1; i < thread_count; ++i) {
+            if (thread_map[i] == to) {
+                tcb_valid = 1;
+                break;
+            }
+        }
+
+        /* Only decrement if TCB is valid AND generation hasn't changed */
+        if (tcb_valid && to->notify_generation == generation_before)
             to->notify_depth--;
-        irq_restore_flags(irq_flags);
+
+        irq_kernel_critical_exit(basepri);
 
         /* Check for preemption after notification.
          * Callback may have made higher-priority threads runnable.

kernel/kdb-latency.c

@@ -0,0 +1,67 @@
+/* Copyright (c) 2026 The F9 Microkernel Project. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include <debug.h>
+#include <platform/irq-latency.h>
+
+/**
+ * KDB command: Display interrupt latency statistics.
+ *
+ * Shows min/avg/max latency for each priority level, highlighting
+ * zero-latency ISRs (0x0-0x2) and standard user IRQs.
+ */
+void kdb_show_latency(void)
+{
+    int i;
+    latency_stats_t stats;
+    int has_data = 0;
+
+    dbg_printf(DL_KDB, "\n=== Interrupt Latency Statistics ===\n");
+    dbg_printf(DL_KDB, "Prio  Type              Count    Min    Avg    Max\n");
+    dbg_printf(DL_KDB, "----  ----------------  ------  -----  -----  -----\n");
+
+    for (i = 0; i < 16; i++) {
+        if (!latency_get_stats_snapshot(i, &stats))
+            continue;
+
+        if (stats.count == 0)
+            continue;
+
+        has_data = 1;
+
+        const char *type;
+        if (i <= 0x2)
+            type = "Zero-latency ISR";
+        else if (i == 0x3)
+            type = "SysTick";
+        else if (i <= 0xE)
+            type = "User IRQ";
+        else
+            type = "SVCall/PendSV";
+
+        stats.avg = stats.count > 0 ? (stats.sum / stats.count) : 0;
+        dbg_printf(DL_KDB, "0x%X   %-16s  %6u  %5u  %5u  %5u\n", i, type,
+                   stats.count, stats.min, stats.avg, stats.max);
+    }
+
+    if (!has_data) {
+        dbg_printf(DL_KDB, "(No latency samples recorded yet)\n");
+    }
+
+    dbg_printf(DL_KDB, "\nNotes:\n");
+    dbg_printf(DL_KDB, "  - Zero-latency ISRs (0x0-0x2) target <10 cycles\n");
+    dbg_printf(DL_KDB, "  - User IRQs (0x4-0xE) masked during kernel ops\n");
+    dbg_printf(DL_KDB, "  - Use 'r' to reset statistics\n");
+    dbg_printf(DL_KDB, "\n");
+}
+
+/**
+ * KDB command: Reset latency statistics.
+ */
+void kdb_reset_latency(void)
+{
+    latency_reset();
+    dbg_printf(DL_KDB, "Latency statistics reset.\n");
+}

kernel/kdb.c

@@ -30,6 +30,8 @@ extern void kdb_dump_as(void);
 extern void kdb_show_sampling(void);
 extern void kdb_show_tickless_verify(void);
 extern void kdb_dump_notifications(void);
+extern void kdb_show_latency(void);
+extern void kdb_reset_latency(void);
 
 struct kdb_t kdb_functions[] = {
     {.option = 'K',
@@ -84,6 +86,14 @@ struct kdb_t kdb_functions[] = {
      .menuentry = "show tickless scheduling stat",
      .function = kdb_show_tickless_verify},
 #endif
+    {.option = 'L',
+     .name = "LATENCY",
+     .menuentry = "show interrupt latency",
+     .function = kdb_show_latency},
+    {.option = 'r',
+     .name = "RESET LATENCY",
+     .menuentry = "reset latency statistics",
+     .function = kdb_reset_latency},
     /* Insert KDB functions here */
 };