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

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Message-ID: <Z6ax7B45OsGerjfA@ghost>
Date: Fri, 7 Feb 2025 17:22:52 -0800
From: Charlie Jenkins <charlie@...osinc.com>
To: Andrew Jones <ajones@...tanamicro.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 05:19:47PM +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? Avoiding a list of hardware that has slow/fast
unaligned accesses in the kernel was the main reason for dynamically
checking. 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.

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.

- Charlie