Re: SBI Debug Console Extension Proposal (Draft v2)


Ved Shanbhogue
 

On Mon, Jun 27, 2022 at 05:38:43PM +0530, Anup Patel wrote:
Also, a system might be partitioned into domains so a supervisor software
running in a particular domain can only pass addresses accessible in that
domain.
So domain is a new term which is not presently defined in the privileged specification or in the SBI specification.

I think I get what you may be stating here i.e., M-mode may configure/
reconfigure PMPs to restrict the physical addresses that are accessible to the supervisor-mode. Perhaps a future Smpu extension may add to this mix.

To avoid inventing a new term, we could reword #1 to state that the physical address must be accessible, as determined by the PMP and/or PMA rules, to the
supervisor mode software invoking this SBI function.

To further restrict this perhaps say "accessible to read"? As we may not want execute-only to be allowed by this extension.


For OpenSBI, we have domain regions so OpenSBI will check the address
against regions of the domain assigned to calling HART.
Since "domain" is a concept that is not defined by either the privilege
specification or the SBI specification we may want to avoid using that
term (or add a definition).

This is more a requirement on the supervisor-mode software because
we might have a system with Svpbmt so on such system supervisor-mode
should not pass an address which is mapped as non-cacheable in the
page table.
This is confusing since the description states that the parameter is a
physical address but the later comment states its a virtual address
and the memory type override using page-based memory type provided by
the virtual memory system is allowed. Could we clarify if its a physical
address - in which case only the coherence and cachability PMA should provide the memory type - or a virtual address in which case page-based memory type override is possible.

Further, a brief explanation about why cachability matters for this extension would be useful. Is this trying to imply some kind of early
boot execution where caches may be placed in a special mode that allows
operation when main memory is not available.

regards
ved

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