Mention that an SBI extension shall not be partially implemented.

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

---

Changes in v2:
- %s/a SBI/an SBI
- reword the clarification

 riscv-sbi.adoc | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/riscv-sbi.adoc b/riscv-sbi.adoc
index 6b548a5..df90840 100644
--- a/riscv-sbi.adoc
+++ b/riscv-sbi.adoc
@@ -35,6 +35,7 @@ https://creativecommons.org/licenses/by/4.0/.
 * Improved documentation of SBI hart state managment extension
 * Added suspend function to SBI hart state managment extension
 * Added performance monitoring unit extension
+* Clarified that an SBI extension shall not be partially implemented
 
 === Version 0.2
 
@@ -52,6 +53,11 @@ abstraction for platform (or hypervisor) specific
functionality. The design
 of the SBI follows the general RISC-V philosophy of having a small
core along
 with a set of optional modular extensions.
 
+SBI extensions as whole are optional but they shall not be partially
+implemented. If sbi_probe_extension() signals that an extension is
available,
+all functions conforming to the SBI version reported by
sbi_get_spec_version()
+must be implemented in total.
+

 The higher privilege software providing SBI interface to the
supervisor-mode
 software is referred to as an SBI implemenation or Supervisor
Execution
 Environment (SEE). An SBI implementation (or SEE) can be platform
runtime