Added TEE SBI Extension description

To support secure service running, cpu running environments
are divided into Trusted execution environment (TEE) and
Rich Execution Environment (REE). This section describes
how to switch between REE and TEE and how to start TEE.

Signed-off-by: liushiwei <[email protected]>

Author: liushiwei

riscv-sbi-tee1.png

@@ -1682,3 +1682,41 @@ Low bits from `mvendorid`.
 Low bits is SBI implementation ID. The firmware specific SBI extensions are
 for SBI implementations. It provides firmware specific SBI functions which
 are defined in the external firmware specification.
+
+== Trusted Execution Environment SBI Extension Space (EID #0x544545 "TEE")
+The Trusted Execution Environment Unit Extension divides cpu Execution
+Environment into two parts: REE (Rich Execution Environment) and TEE
+(Trusted execution environment), As shown in the picture below <<fig_tee1>>.
+To enable some applications to perform security-related services,
+which are placed in TEE.
+
+[#fig_tee1]
+.SBI TEE extensions runtimes
+image::riscv-sbi-tee1.png[width=1007,height=464]
+
+Well, one option is REE runs linux, TEE runs optee-os.
+When starting a security service, REE needs to convey 8 parameters
+to TEE according to optee-os standard,
+and TEE needs to return 4 parameters to REE when TEE is finished.
+So linux needs to pass 10 parameters to opensbi. I use the a0-a7,
+t0, t1 register. the newly added correlation parameters,
+one conveying 0x544545 indicating that this is a TEE extension,
+and the other conveying the caller status indicating
+it is from REE or from TEE. Unlike other SBI extensions contexts saved,
+In the tee process, not only sbi_trap_regs but also CSRs of S mode
+should be saved including all calls from REE and some calls from TEE.
+
+REE S mode CSRs is derived from linux, and TEE S mode CSRs is derived
+from optee os initialization. Upon startup, opensbi will add
+a tee_os_init function before sbi_hart_switch_mode.
+tee_os_init will jump to optee os for initialization.
+The startup address of optee os is configured using configuration items.
+After the initialization is complete, it returns to opensbi.
+The return parameter holds the optee os entry address of the runtime,
+and it gets EE S mode CSRs. Then go back to the previous execution process
+to start linux. The same function is used to boot the secondary hart.
+The following figure(<<fig_tee2>>) shows the startup process.
+
+[#fig_tee2]
+.SBI TEE extensions boot flow
+image::riscv-sbi-tee2.png[width=975,height=527]