linux-kernel - Re: [PATCH v2] x86/cpu: Enable SD_ASYM_PACKING for PKG domain on systems with AMD preferred cores

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Message-ID: <20241203221746.GKZ0-Dii5rnZppkM_e@fat_crate.local>
Date: Tue, 3 Dec 2024 23:17:46 +0100
From: Borislav Petkov <bp@...en8.de>
To: Mario Limonciello <mario.limonciello@....com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
	"H . Peter Anvin" <hpa@...or.com>,
	"open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <linux-kernel@...r.kernel.org>,
	Perry Yuan <Perry.Yuan@....com>,
	Shyam Sundar S K <Shyam-sundar.S-k@....com>
Subject: Re: [PATCH v2] x86/cpu: Enable SD_ASYM_PACKING for PKG domain on
 systems with AMD preferred cores

On Tue, Dec 03, 2024 at 04:02:29PM -0600, Mario Limonciello wrote:
> Preferred core classifications are available on "non-heterogenous" designs
> for a few generations.  There isn't an indication they're supported which is
> why amd_detect_prefcore() was made.

Not surprised.

> That's already called during the boot either way because that is used
> to identify the boost numerator.  The boolean value it finds is cached, and
> the next call will use the cached value.  So I don't expect this change
> affects boot speed.

So with this addition, amd_detect_prefcore() would get called *three* times
where you need to call it exactly and only once. And all it gives you a one
bit of information which states that the system has preferred cores.

So why don't you define a synthetic X86_FEATURE:

	X86_FEATURE_AMD_PREFERRED_CORES

run this code *once* and *early* enough so that every user can have the result
ready, if you have preferred cores, do

	setup_force_cpu_cap(X86_FEATURE_AMD_PREFERRED_CORES)

(it needs to run before alternatives) and then in each user, you do

	if (cpu_feature_enabled(X86_FEATURE_AMD_PREFERRED_CORES))
		/* do things */

instead of what you have now?

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette