Re: [PATCH v2 4/5] x86/cpu: Add CPU type to struct cpuinfo_topology

[Mario Limonciello <mario.limonciello@amd.com>]

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?