What is the best way to setup a core to be visible in M-mode but not by an operating system in S/U-mode?

[chungmcl]

I am currently contributing to Keystone (https://github.com/keystone-enclave/keystone) which utilizes OpenSBI, and we are currently trying to achieve this behavior:

We want to have a system with a "management core" such that this core is visible to the M-mode supervisor, but not to anything else (i.e. an operating system and its applications in S/U-mode).

We are not quite sure how to best achieve this with OpenSBI as it is currently -- do you guys have any thoughts on how best to achieve this? We've made some modifications in our private fork of OpenSBI, but we would of course like to stick with the standard release of OpenSBI as much as possible. We are also thinking this may need the changes we suggested in this issue: https://github.com/riscv-software-src/opensbi/issues/297

[GrootLiu]

Do you want to set a separate core to serve as a privileged delegation?