Re: SBI Debug Console Extension Proposal (Draft v2)


Anup Patel
 

On Mon, Jun 27, 2022 at 6:16 PM Ved Shanbhogue <ved@...> wrote:

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.
The term "domain" is not used in the proposal since it is OpenSBI specific.
My comment was mainly for Heinrich since he is already aware of OpenSBI
domains.

In general, other M-mode firmwares might also implement some kind of
system level partitioning.


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.
I mostly agree with your wording. Considering hypervisors and future memory
protection ISA extensions, let's avoid using terms PMP.

For #1, we can say:
The SBI implementation MUST check that the supervisor-mode software
is allowed to read the specified physical address range on the calling HART.



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).
The term "domain" was only in the context of OpenSBI. We should not use
this term in the SBI specification.


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.
I agree the #2 requirement is not clear enough.

For #2, we can say:
The supervisor-mode software MUST not override the memory type of
specified physical address range using page-based memory types.

The rationale is that for M-mode the memory type is always defined by
PMA(s) so if supervisor-mode software overrides memory type using
Svpbmt then M-mode firmware will not see same contents as the
supervisor-mode software.

Regards,
Anup

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