lib: sbi: Add support for Smepmp

- If Smepmp is enabled, the access flags of an entry are determined based on
  truth table defined in Smepmp.
- First PMP entry (index 0) is reserved.
- Existing boot PMP entries start from index 1.
- Since enabling Smepmp revokes the access privileges of the M-mode software
  on S/U-mode region, first PMP entry is used to map/unmap the shared memory
  between M and S/U-mode. This allows a temporary access window for the M-mode
  software to read/write to S/U-mode memory region.

Signed-off-by: Himanshu Chauhan <hchauhan@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>

include/sbi/sbi_hart.h

@@ -45,6 +45,21 @@ enum sbi_hart_extensions {
 	SBI_HART_EXT_MAX,
 };
 
+/*
+ * Smepmp enforces access boundaries between M-mode and
+ * S/U-mode. When it is enabled, the PMPs are programmed
+ * such that M-mode doesn't have access to S/U-mode memory.
+ *
+ * To give M-mode R/W access to the shared memory between M and
+ * S/U-mode, first entry is reserved. It is disabled at boot.
+ * When shared memory access is required, the physical address
+ * should be programmed into the first PMP entry with R/W
+ * permissions to the M-mode. Once the work is done, it should be
+ * unmapped. sbi_hart_map_saddr/sbi_hart_unmap_saddr function
+ * pair should be used to map/unmap the shared memory.
+ */
+#define SBI_SMEPMP_RESV_ENTRY		0
+
 struct sbi_hart_features {
 	bool detected;
 	int priv_version;