Re: [PATCH 7/9] riscv: Prepare for unaligned access type table lookups

[Andrew Jones]

On Fri, Feb 07, 2025 at 05:22:52PM -0800, Charlie Jenkins wrote:
> On Fri, Feb 07, 2025 at 05:19:47PM +0100, Andrew Jones wrote:
> > Probing unaligned accesses on boot is time consuming. Provide a
> > function which will be used to look up the access type in a table
> > by id registers. Vendors which provide table entries can then skip
> > the probing.
> 
> The access checker in my experience is only time consuming on slow
> hardware. Hardware that supports fast unaligned accesses isn't really
> impacted by this?

That's true, but...

> Avoiding a list of hardware that has slow/fast
> unaligned accesses in the kernel was the main reason for dynamically
> checking.

...I'm not sure why we should try to avoid determining hardware support
by its description when a description can be provided.

> We did introduce the config option to compile the kernel with
> assumed slow/fast accesses, which of course has the downside of
> recompiling the kernel and I assume that you already considered that.

yup

> 
> Instead of having a table in the kernel, something that would be more
> platform agnostic would be to have an extension that signals this
> information. That seems like it would accomplish the same goal and
> leverage the existing infrastructure in the kernel, albeit with the need
> to make a new extension.

Yes, I agree that another profile "named feature" may be the best
approach. I'll consider proposing one, but [1] implies there may be
some resistance to creating something like that.

[1] https://github.com/riscv/riscv-isa-manual/issues/1611

Thanks,
drew