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Clean up

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Lee Lup Yuen 2023-07-07 14:43:34 +08:00
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@ -1256,6 +1256,8 @@ NuttX runs at Supervisor Level, which [doesn't allow access to Machine-Level CSR
(The `m` in `mhartid` signifies that it's a Machine-Level Register)
![RISC-V Privilege Levels](https://lupyuen.github.io/images/nuttx2-privilege.jpg)
_What runs at Machine Level?_
[OpenSBI](https://www.thegoodpenguin.co.uk/blog/an-overview-of-opensbi/) (Supervisor Binary Interface) is the first thing that boots on Star64. It runs at Machine Level and starts the U-Boot Bootloader.
@ -1268,6 +1270,14 @@ U-Boot Bootloader runs at Supervisor Level. And starts NuttX, also at Supervisor
So OpenSBI is the only thing that runs at Machine Level. And can access the Machine-Level Registers.
_QEMU doesn't have this problem?_
Because QEMU runs everything in (super-powerful) __Machine Mode__!
![NuttX QEMU runs in Machine Mode](https://lupyuen.github.io/images/nuttx2-privilege2.jpg)
NuttX needs to fetch the Hart ID with a different recipe...
# Downgrade NuttX to Supervisor Mode
_How to get the Hart ID from OpenSBI?_