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

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Message-ID: <20250210-51cedc94f4cba8c96516adc5@orel>
Date: Mon, 10 Feb 2025 10:43:18 +0100
From: Andrew Jones <ajones@...tanamicro.com>
To: Charlie Jenkins <charlie@...osinc.com>
Cc: linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	paul.walmsley@...ive.com, palmer@...belt.com, jesse@...osinc.com, 
	Anup Patel <apatel@...tanamicro.com>
Subject: Re: [PATCH 7/9] riscv: Prepare for unaligned access type table
 lookups

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