RVA_020: Provide additional context for Ssccfg

[punitagrawal]

The privileged specification includes a chapter about "Smcdeleg" Counter Delegation Extension, Version 1.0.0 that includes the description of Smcdeleg and Ssccfg.

For clarity / ease of finding the the details, update the RVA_020 requirement to include a reference to Smcdeleg.

Closes: #19

[ved-rivos]

This was intentional. The Platform specification is written to provide the requirements that the privilege levels less than M can expect. We have not placed any mandates on the M-mode ISA in the Server Platform or the Server SoC specification. I notice we have Smcntrpmf which I think should also be removed.

[ved-rivos]

The smcdeleg and smcntrpmf should be remove from the list.

[punitagrawal]

@ved-rivos Thank you for taking a look.

The reason for adding smcdeleg was that there is confusion about ssccfg - folks have complained about not being able to find its description. Even the chapter describing it only includes smcdeleg. I've seen ssccfg being incorrectly referred to in many places as sscfg (one example[0]) which obviously then makes it even harder to find.

I just had a look to confirm the content of smcntrpmf - it seems to be implied if smcdeleg/ssccfg is implemented. So dropping it should be ok. Though I hope this indirect requirement doesn't get missed by any implementation.

For my learning - I was wondering what is the rationale for the platform specification targeting privilege levels less than M. Is there a downside to having M mode extensions also in the list?

[ved-rivos]

For my learning - I was wondering what is the rationale for the platform specification targeting privilege levels less than M. Is there a downside to having M mode extensions also in the list?

The RVA profiles, server SoC specification, and the server platform specification are written to specify requirements as perceived by the S-mode and lower. This is to ensure we can support a single binary OS distribution on compliant platforms. An implementation that claims support Ssccfg should provide that extension as perceived by the S-mode. We are presently not placing mandates on the M-mode ISA - in fact none of these specifications even require the M-mode ISA to be implemented - for instance the machine mode may be an emulator.

[punitagrawal]

The smcdeleg and smcntrpmf should be remove from the list.

I've pushed an updated patch doing this.