Introduce Physical Memory Protection Extension

S-mode software needs a way to know memory used by SBI firmware so
that it can correctly mark such memory as reserved.

Related discussion:
riscv-software-src/opensbi#103

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>

riscv-sbi.adoc

@@ -592,6 +592,52 @@ state of the hart at the time of return value verification.
 | sbi_hart_get_status      	|           2 |     0x48534D
 |===
 
+== Physical Memory Protection Extension, Extension ID: 0x504D50 (PMP)
+The Physical Memory Protection Extension introduces a set of functions that
+allow the supervisor to request higher privilege mode to return the PMP
+protected SBI firmware information.
+
+[source, C]
+----
+struct sbiret sbi_get_fw_prot_base()
+----
+*Returns* the protected SBI firmware base physical address through sbiret.value,
+or one of the following possible SBI error codes through sbiret.error.
+
+[cols="<,>",options="header,compact"]
+|===
+| Error code                | Description
+| SBI_SUCCESS               | SBI firmware is protected via PMP, and its base +
+                              physical address is returned through sbiret.value.
+| SBI_ERR_NOT_SUPPORTED     | PMP is not available on this platform, or SBI +
+                              firmware is not protected via PMP.
+|===
+
+[source, C]
+----
+struct sbiret sbi_get_fw_prot_size()
+----
+*Returns* the protected SBI firmware size through sbiret.value, or one of the
+following possible SBI error codes through sbiret.error.
+
+[cols="<,>",options="header,compact"]
+|===
+| Error code                | Description
+| SBI_SUCCESS               | SBI firmware is protected via PMP, and its size +
+                              is returned through sbiret.value.
+| SBI_ERR_NOT_SUPPORTED     | PMP is not available on this platform, or SBI +
+                              firmware is not protected via PMP.
+|===
+
+=== PMP Function Listing
+
+[cols="<,,>",options="header,compact"]
+|===
+| Function Name             | Function ID | Extension ID
+| sbi_get_fw_prot_base      |           0 |     0x504D50
+| sbi_get_fw_prot_size      |           1 |     0x504D50
+|===
+
 == Experimental SBI Extension Space, Extension IDs 0x0800000 through 0x08FFFFFF
 
 No management.