Re: [patches] Re: [PATCH] dt-bindings: Add a RISC-V SBI firmware node

[Palmer Dabbelt]

On Mon, 20 Nov 2017 17:08:44 PST (-0800), j.neuschaefer@xxxxxxx wrote:
> On Mon, Nov 20, 2017 at 01:28:01PM -0800, Palmer Dabbelt wrote:
> > > > @@ -0,0 +1,20 @@
> > > > +RISC-V Supervisor Binary Interface (SBI)
> > > > +
> > > > +The RISC-V privileged ISA specification mandates the presence of a supervisor
> > > > +binary interface that performs some operations which might otherwise require
> > > > +particularly complicated instructions.  This interface includes
> > > > +inter-processor interrupts, TLB flushes, i-cache and TLB shootdowns, a
> > > > +console, and power management.
> > > > +
> > > > +Required properties:
> > > > +- compatible: must contain one of the following
> > > > + * "riscv,sbi" for the SBI defined by the privileged specification of the
> > > > +   system.
> > > 
> > > "of the system" seems to imply that different RISC-V systems (different
> > > RISC-V implementations) can have different privileged specifications.
> >
> > Actually, that was intentional -- I wrote it this way because different
> > RISC-V systems do have different privileged specifications.  The RISC-V
> > specifications aren't frozen in time, they're just guaranteed to be
> > compatible in the future.  For example, the user ISA document has been
> > updated multiple times (the C spec, eliminating some unspecified behavior)
> > and will continue to be updated (V and other extensions, the memory model).
> > The privileged spec will be updated in a compatible way just like the user
> > spec will be -- I know there's at least hypervisor support in the works, and
> > I saw some things to remove undefined behavior go past as well.
> >
> > In a similar fashion, the ABI and SBI will continue to evolve.  For example,
> > we'll probably add new system calls to extend the user ABI and new hyper
> > calls to extend the SBI.
>
> My problem with the wording was that the OS somehow has to know which
> version and variant of the SBI it is talking to -- either through
> in-band communication (an SBI call to request SBI information, etc.), or
> through devicetree or similar mechanisms.

The idea here is that rather than checking for a version of the SBI, you just 
check for various features as you need them.