[GitHub] [incubator-nuttx] luojia65 commented on pull request #5782: RISC-V: Implement option to run NuttX in supervisor mode (S-mode)

[GitBox <gi...@apache.org>]

luojia65 commented on PR #5782:
URL: https://github.com/apache/incubator-nuttx/pull/5782#issuecomment-1306777999

   > There is one big issue with the spec. IMO, as it does not provide a way to read the timer (mtime) value or the value of mtimecmp. To create an accurate and safe to use ticker both are needed, as there is no way to calculate time in absolute format otherwise. If it was an offset then it would make sense, but it's not. This ABI does not really make any sense to me.
   
   @pussuw at https://github.com/apache/incubator-nuttx/pull/5782#issuecomment-1080247601 : The SBI environment is typically responsible for RISC-V software register `time`. RISC-V registers include two parts, one implemented by RISC-V hardware and the other part defined by software. RISC-V machine environments typically use trap-and-return procedure to emulate software registers. The register `time` is in absolute, meaning that RISC-V supervisor software may read `time` directly, add difference of time and fill it using SBI set_time. Here's some assemble code:
   
   ```asm
   csrr t0, time ; read time register from SBI environment
   li t1, 1000000
   add t0, t0, t1 ; calculate the next time it should interrupt supervisor
   mv a0, t0
   li a6, FUNCTION_SET_TIME
   li a7, EXTENSION_TIMER
   ecall ; call SBI environment to set timer
   ```
   
   Similiar software emulated RISC-V register would include CSR `cycle`.


-- 
This is an automated message from the Apache Git Service.
To respond to the message, please log on to GitHub and use the
URL above to go to the specific comment.

To unsubscribe, e-mail: commits-unsubscribe@nuttx.apache.org

For queries about this service, please contact Infrastructure at:
users@infra.apache.org