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: <2288034e-7819-4327-852c-8c00b92a1493@amd.com>
Date: Tue, 3 Dec 2024 16:21:20 -0600
From: Mario Limonciello <mario.limonciello@....com>
To: Borislav Petkov <bp@...en8.de>
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 12/3/2024 16:17, Borislav Petkov wrote:
> 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.
> 

Sure, that sounds reasonable, I'll try to find the most appropriate call 
site for it.