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

[Palmer Dabbelt]

On Wed, 31 Oct 2018 04:16:10 PDT (-0700), anup@xxxxxxxxxxxxxx wrote:
> On Wed, Oct 31, 2018 at 4:06 PM Vincent Chen <vincentc@xxxxxxxxxxxxx> wrote:
> >   RISC-V permits each vendor to develop respective extension ISA based
> > on RISC-V standard ISA. This means that these vendor-specific features
> > may be compatible to their compiler and CPU. Therefore, each vendor may
> > be considered a sub-architecture of RISC-V. Currently, vendors do not
> > have the appropriate examples to add these specific features to the
> > kernel. In this RFC set, we propose an infrastructure that vendor can
> > easily hook their specific features into kernel. The first commit is
> > the main body of this infrastructure. In the second commit, we provide
> > a solution that allows dma_map_ops() to work without cache coherent
> > agent support. Cache coherent agent is unsupported for low-end CPUs in
> > the AndeStar RISC-V series. In order for Linux to run on these CPUs, we
> > need this solution to overcome the limitation of cache coherent agent
> > support. Hence, it also can be used as an example for the first commit.
> >
> >   I am glad to discuss any ideas, so if you have any idea, please give
> > me some feedback.
>
> I agree that we need a place for vendor-specific ISA extensions and
> having vendor-specific directories is also good.
>
> What I don't support is the approach of having compile time selection
> of vendor-specific ISA extension.
>
> We should have runtime probing for compatible vendor-specific ISA
> extension. Also, it should be possible to link multiple vendor-specific
> SA extensions to same kernel image. This way we can have a single
> kernel image (along with various vendor-specific ISA extensions) which
> works on variety of targets/hosts.
>
> As an example or runtime probing you can look at how IRQCHIP or
> CLOCKSOURCE drivers are probed. The vendor-specific ISA extension
> hooks should called in similar fashion.

Yes, I agree.  My biggest concern here is that we ensure that one kernel can 
boot on implementations from all vendors.  I haven't had a chance to look at 
the patches yet, but it should be possible to:

* Build a kernel that has vendor-specific code from multiple vendors.
* Detect the implementation an run time and select the correct extra code.

This is essentially the same as my feedback for the performance counter stuff, 
which IIRC is what prompted adding a vendor-specific extensions.

If I was going to do this, I'd split it up such that the vendor-specific 
additions are per-subsystem.  That way we can focus on building a decent 
interface for each subsystem that needs vendor-specific support rather than 
just bundling everything together where the vendor-specific stuff will get all 
tangled together.