add subsection of indirect CSR access

spmp_spec.adoc

@@ -207,4 +207,40 @@ Each bit of this register holds the on/off status of the corresponding SPMP entr
 During the context switch, the OS can store and restore spmpswitch as part of the context.
 An SPMP entry is activated only when both corresponding bits in spmpswitch and A field of spmp[i]cfg are set. (i.e., spmpswitch[i] & spmp[i]cfg.A!=0)
 
-image::SPMP_domain_switch_register_format.svg[title="SPMP domain switch register format (RV64)"]
+image::SPMP_domain_switch_register_format.svg[title="SPMP domain switch register format (RV64)"]
+
+
+
+
+=== Access Methods of SPMP CSRs
+*Indirect CSR access*: The SPMP support indirect CSR access if the `Sscsrind` extension is implemented.
+The `Sscsrind` defines 1 select CSR (`siselect`) and 6 alias CSRs (`sireg[i]`).
+The each combination of `siselect` and `sireg[i]` standards for an access of corresponding SPMP CSR.
+
+[cols="^1,^2",stripes=even, options="header"]
+|===
+|siselect number|indirect CSR access of sireg[i]
+|0x1a0|sireg[1-6] -> spmpcfg[0-5]
+|0x1a1|sireg[1-6] -> spmpcfg[6-11]
+|0x1a2|sireg[1-4] -> spmpcfg[12-15]
+|0x1a3|sireg[1-6] -> spmpaddr[0-5]
+|0x1a4|sireg[1-6] -> spmpaddr[6-11]
+|0x1a5|sireg[1-6] -> spmpaddr[12-17]
+|0x1a6|sireg[1-6] -> spmpaddr[18-23]
+|0x1a7|sireg[1-6] -> spmpaddr[24-29]
+|0x1a8|sireg[1-6] -> spmpaddr[30-35]
+|0x1a9|sireg[1-6] -> spmpaddr[36-41]
+|0x1aa|sireg[1-6] -> spmpaddr[42-47]
+|0x1ab|sireg[1-6] -> spmpaddr[48-53]
+|0x1ac|sireg[1-6] -> spmpaddr[54-59]
+|0x1ad|sireg[1-4] -> spmpaddr[60-63]
+|0x1ae|sireg[1-2] -> spmpswitch[0-1]
+|===
+
+*Direct CSR access*: SPMP CSRs can be accessed directly with corresponding CSR numbers if the `Sscsrind` extension is not implemented.
+
+[NOTE]
+====
+Please refers to the specification of the `Sscsrind` extension for details of indirect CSR access.
+https://github.com/riscv/riscv-indirect-csr-access
+====