arch: riscv: Add function to clear all unlocked PMP entries

Introduce the new function `riscv_pmp_clear_all()` to securely reset
the Physical Memory Protection (PMP) configuration.

This function iterates through all configured PMP slots. For each entry,
it checks if the Lock (L) bit is set. If the entry is not locked,
it clears the Address Matching Mode (A) bits, effectively setting the
region type to OFF (Null Region), disabling the entry.

The function ensures it operates in Machine mode with MSTATUS.MPRV = 0
before modifying any PMP Control and Status Registers (CSRs).

This provides a mechanism to clear all non-locked PMP regions,
returning them to a default disabled state. The function is exposed
in the pmp.h header file.

Signed-off-by: Firas Sammoura <[email protected]>

Author: fsammoura1980

arch/riscv/core/pmp.c

@@ -345,6 +345,54 @@ static unsigned long global_pmp_last_addr;
 /* End of global PMP entry range */
 static unsigned int global_pmp_end_index;
 
+void riscv_pmp_clear_all(void)
+{
+	/*
+	 * Ensure we are in M-mode and that memory accesses use M-mode privileges
+	 * (MPRV=0). We also set MPP to M-mode to establish a predictable prior privilege level.
+	 */
+	csr_clear(mstatus, MSTATUS_MPRV);
+	csr_set(mstatus, MSTATUS_MPP);
+
+	unsigned long pmp_cfg[CONFIG_PMP_SLOTS / PMPCFG_STRIDE];
+
+#ifdef CONFIG_64BIT
+	pmp_cfg[0] = csr_read(pmpcfg0);
+#if CONFIG_PMP_SLOTS > 8
+	pmp_cfg[1] = csr_read(pmpcfg2);
+#endif
+#else
+	pmp_cfg[0] = csr_read(pmpcfg0);
+	pmp_cfg[1] = csr_read(pmpcfg1);
+#if CONFIG_PMP_SLOTS > 8
+	pmp_cfg[2] = csr_read(pmpcfg2);
+	pmp_cfg[3] = csr_read(pmpcfg3);
+#endif
+#endif
+
+	uint8_t *pmp_n_cfg = (uint8_t *)pmp_cfg;
+
+	for (int index = 0; index < CONFIG_PMP_SLOTS; ++index) {
+		if ((pmp_n_cfg[index] & PMP_L) == 0) {
+			pmp_n_cfg[index] &= ~PMP_A;
+		}
+	}
+
+#ifdef CONFIG_64BIT
+	csr_write(pmpcfg0, pmp_cfg[0]);
+#if CONFIG_PMP_SLOTS > 8
+	csr_write(pmpcfg2, pmp_cfg[1]);
+#endif
+#else
+	csr_write(pmpcfg0, pmp_cfg[0]);
+	csr_write(pmpcfg1, pmp_cfg[1]);
+#if CONFIG_PMP_SLOTS > 8
+	csr_write(pmpcfg2, pmp_cfg[2]);
+	csr_write(pmpcfg3, pmp_cfg[3]);
+#endif
+#endif
+}
+
 /**
  * @Brief Initialize the PMP with global entries on each CPU
  */

arch/riscv/include/pmp.h

@@ -15,4 +15,15 @@ void z_riscv_pmp_usermode_init(struct k_thread *thread);
 void z_riscv_pmp_usermode_prepare(struct k_thread *thread);
 void z_riscv_pmp_usermode_enable(struct k_thread *thread);
 
+/**
+ * @brief Resets all unlocked PMP entries to OFF mode (Null Region).
+ *
+ * This function is used to securely clear the PMP configuration. It first
+ * ensures the execution context is M-mode by setting MSTATUS_MPRV=0 and
+ * MSTATUS_MPP=M-mode. It then reads all pmpcfgX CSRs, iterates through
+ * the configuration bytes, and clears the Address Matching Mode bits (PMP_A)
+ * for any entry that is not locked (PMP_L is clear), effectively disabling the region.
+ */
+void riscv_pmp_clear_all(void);
+
 #endif /* PMP_H_ */