v01d opened a new issue #3424:
URL: https://github.com/apache/incubator-nuttx/issues/3424


   ```
     Description: Couldn't up_start_task and up_start_pthread syscalls be
                  eliminated.  Wouldn't this work to get us from kernel-
                  to user-mode with a system trap:
   
                    lda r13, #address
                    str rn, [r13]
                    msr spsr_SVC, rm
                    ld r13,{r15}
   
                  Would also need to set r13_USER and r14_USER. For new
                  SYS_context_switch... couldn't we do he same thing?
   
                  Also... System calls use traps to get from user- to kernel-
                  mode to perform OS services.  That is necessary to get from
                  user- to kernel-mode.  But then another trap is used to get
                  from kernel- back to user-mode.  It seems like this second
                  trap should be unnecessary.  We should be able to do the
                  same kind of logic to do this.
     Status:      Open
     Priority:    Low-ish, but a good opportunity for performance improvement.
   ```


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