linux-kernel - Re: [PATCH 2/4] arm64/fpsimd: Discover maximum vector length implemented by any CPU

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Message-ID: <50d3d9d3-50f2-4c95-a481-6e744ae17099@sirena.org.uk>
Date: Wed, 5 Jun 2024 13:56:19 +0100
From: Mark Brown <broonie@...nel.org>
To: Fuad Tabba <tabba@...gle.com>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>,
	James Morse <james.morse@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	kvmarm@...ts.linux.dev
Subject: Re: [PATCH 2/4] arm64/fpsimd: Discover maximum vector length
 implemented by any CPU

On Wed, Jun 05, 2024 at 01:03:16PM +0100, Fuad Tabba wrote:

> Actually, I was working on fixing it and was about to send this, which
> I think might be a bit simpler than what you have. Let me know what
> you think and I'll send it as a proper patch if you agree:

> +
> +               if (!max_vl)
> +                       max_vl = vl;
>         }
> +
> +       info->max_system_vl = max((int) max_vl, info->max_system_vl);

Yeah, I originally had written something like this (just doing the max
in the original assignment rather than staging in a local variable) but
it didn't feel great and like it was more vulnerable to getting missed
if we do more parallel bringup so I switched to keeping the split
between enumeration and integration.

I do also prefer the _cpu_ naming since _system_ could be read as the
maximum that we're actually using in the system, as with any naming
thing it's all a bit bikesheddy.

>         if (system_supports_sve()) {
>                 kvm_sve_max_vl = sve_max_virtualisable_vl();
> -               kvm_host_sve_max_vl = sve_max_vl();
> +               kvm_host_sve_max_vl = vl_info[ARM64_VEC_SVE].max_system_vl;

We should keep the accessor functions, it's more consistent and supports
refactoring.

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