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SubjectRe: [RFC 0/2] RISC-V: A proposal to add vendor-specific code
On Thu, Nov 01, 2018 at 10:50:04AM -0700, Palmer Dabbelt wrote:
> On Wed, 31 Oct 2018 17:55:42 PDT (-0700), alankao@andestech.com wrote:
> >On Wed, Oct 31, 2018 at 07:17:45AM -0700, Christoph Hellwig wrote:
> >>On Wed, Oct 31, 2018 at 04:46:10PM +0530, Anup Patel wrote:
> >>> I agree that we need a place for vendor-specific ISA extensions and
> >>> having vendor-specific directories is also good.
> >>
> >>The only sensible answer is that we should not allow vendor specific
> >>extensions in the kernel at all. ...
> >
> >How can this even be possible if a extension includes an extra register
> >set as some domain-specific context? In such a case, kernel should
> >at least process the context during any context switch, just like how it
> >deals with the FP context.
>
> Ya, I think there are cases where vendor-specific extensions are going to be
> necessary to handle within the kernel. Right now the only one I can think
> of is the performance counter stuff, where we explicitly allow
> vendor-specific counters as part of the ISA spec.
>
> For stateful extensions, we currently have a standard mechanism where the XS
> bits get set in sstatus and the actual save/restore code is hidden behind an
> SBI call. That call doesn't currently exist, but if we just go ahead and
> add one it should be easy to support this from within Linux. We'll need to
> figure out how to enable these custom extensions from userspace, but that
> seems tractable as well. We'll probably also want some fast-path for the V
> extension (and any other stateful standard extensions), but I think as long
> as the V extension adds a quick check for dirtiness then it's not a big
> deal.
>
> Do you guys have stateful extensions? We're trying really hard to avoid
> them at SiFive because they're a huge headache, so unless there's a
> compelling base of software using one I don't want to go add support if we
> can avoid it.

Currently no, but the future is hard to see. As long as the extensible freedom
claimed by the RISC-V foundation remains true, such extensions may have their
role to play. Don't worry now, I was just to give a example that in some
possible vendor-specific cases the kernel cannot keep itself from involving.

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