linux-kernel - Re: [PATCH v2 4/5] x86/cpu: Add CPU type to struct cpuinfo

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Message-ID: <13fd2271-f64e-4573-afdb-9881b8c399fe@amd.com>
Date: Tue, 22 Oct 2024 11:13:00 -0500
From: Mario Limonciello <mario.limonciello@....com>
To: Dave Hansen <dave.hansen@...el.com>, 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>, "Rafael J . Wysocki" <rafael@...nel.org>,
 "Gautham R . Shenoy" <gautham.shenoy@....com>,
 Perry Yuan <perry.yuan@....com>, Brijesh Singh <brijesh.singh@....com>,
 Peter Zijlstra <peterz@...radead.org>, Li RongQing <lirongqing@...du.com>,
 "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
 <linux-kernel@...r.kernel.org>, "open list:ACPI"
 <linux-acpi@...r.kernel.org>,
 "open list:AMD PSTATE DRIVER" <linux-pm@...r.kernel.org>,
 Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Subject: Re: [PATCH v2 4/5] x86/cpu: Add CPU type to struct cpuinfo_topology

On 10/22/2024 11:03, Dave Hansen wrote:
> On 10/22/24 04:57, Borislav Petkov wrote:
>> +enum x86_topology_cpu_type get_intel_cpu_type(struct cpuinfo_x86 *c)
>> +{
>> +	switch (c->topo.intel_type) {
>> +	case 0x20: return TOPO_CPU_TYPE_EFFICIENCY;
>> +	case 0x40: return TOPO_CPU_TYPE_PERFORMANCE;
>> +	}
>> +	return TOPO_CPU_TYPE_UNKNOWN;
>> +}
> 
> This makes me feel a _bit_ uneasy.  0x20 here really does mean "Atom
> microarchitecture" and 0x40 means "Core microarchitecture".
> 
> We want to encourage folks to use this new ABI when they want to find
> the fastest core to run on.  But we don't want them to use it to bind to
> a CPU and then deploy Atom-specific optimizations.
> 
> We *also* don't want the in-kernel code to do be doing things like:
> 
> 	if (get_intel_cpu_type() == TOPO_CPU_TYPE_EFFICIENCY)
> 		setup_force_cpu_bug(FOO);
> 
> That would fall over if Intel ever mixed fast and slow core types with
> the same microarchitecture, which is what AMD is doing today.
> 
> Having:
> 
> 	TOPO_CPU_TYPE_EFFICIENCY, and
> 	TOPO_CPU_TYPE_PERFORMANCE
> 
> is totally fine in generic code.  But we also need to preserve the:
> 
> 	TOPO_HW_CPU_TYPE_INTEL_ATOM
> 	TOPO_HW_CPU_TYPE_INTEL_CORE
> 
> values also for use in vendor-specific code.

What you're suggesting is to keep an enum in the intel.c code and any 
code that needs to match atom vs core can directly use

c->topo.intel_type == TOPO_HW_CPU_TYPE_INTEL_ATOM

Right?

> 
> In the ABI, I think we should probably make this an explicit
> power/performance interface rather than "cpu_type".  As much as I like
> the human readable "performance" and "efficiency", I'm worried it won't
> be flexible enough for future maniacal hardware designers.  To be 100%
> clear, all the hardware designs that I know of would fit in a two-bucket
> ("performance" and "efficiency") scheme.  But we've got to decide
> whether to commit to that forever.

As it stands today none of this is exported anywhere but debugfs; so I 
wouldn't say we have ABI concerns (yet).  Could we wait until the one 
that breaks the mold shows up?