[GitHub] [incubator-nuttx] xiaoxiang781216 edited a comment on pull request #5782: RISC-V: Implement option to run NuttX in supervisor mode (S-mode)

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xiaoxiang781216 edited a comment on pull request #5782:
URL: https://github.com/apache/incubator-nuttx/pull/5782#issuecomment-1076299011


   > > S-mode can hook ECALL to self, @pussuw could you explain more why we need handle differently in S-mode and M-mode?
   > 
   > S-mode can trap ecall from user mode (U-mode) but not from S-mode. So ecall from S-mode to S-mode is not possible.
   
   Ok, I get the key point. So is syscall from S-mode to S-mode only used for kernel thread context switch?
   
   > Also, we want to use ecall in S-mode to raise privileges to M-mode, to access some machine mode services (mhartid,
   
   Yes, but it's still the same ecall instruction. Here is my understanding:
   
   1. Any syscall(include context switch) issued by U-mode could be implemented by ECALL and raise to S-mode
   2. Kernel thread context switch could be implemented by ECALL too, but it will be handled in M-mode instead S-mode
   3. Some special function(e.g. mhartid) which need the highest privilege has to issue ECALL from S-mode to M-mode
   
   So, I think we can still use the same sys_call0-syscall6 as before, and:
   
   1. Handle U-mode syscall in S-mode as before
   2. Handle S-mode context switch in M-mode( the code should be same as S-mode)
   3. Extend the reserved syscall id to cover the special need(e.g. mhardid)
   
   > mtimer to name a few, see riscv_mcall.c).
   
   Why we can't use S-mode timer instead?
   


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