Re: [RFC 0/2] RISC-V: A proposal to add vendor-specific code

[Christoph Hellwig]

On Mon, Nov 05, 2018 at 09:52:52AM +0100, Arnd Bergmann wrote:
> > I fundamentally disagree with thisâ and think it should be the contrary.
> >
> > 1. The kernel shall support no vendor specific instructions whatsoever,
> > period.
> 
> I think what was meant above is
> 
> 1. If a vendor extension requires kernel support, that support
> must be able to be built into a kernel image without breaking support
> for CPUs that do not have that extension, to allow building a single
> kernel image that works on all CPUs.

No.  This literally means no vendor extensions involving instructions
or CSRs in the kernel.  They are fine for userspace, or for the M-mode
code including impementation of the SBI, but not for the kernel.