linux-kernel - Re: [PATCH 0/3] x86: Add initial support to discover Intel hybrid CPUs

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Date:   Sat, 3 Oct 2020 11:04:29 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        x86@...nel.org, Ingo Molnar <mingo@...nel.org>,
        Len Brown <len.brown@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: Re: [PATCH 0/3] x86: Add initial support to discover Intel hybrid
 CPUs

On Fri, Oct 02, 2020 at 07:17:30PM -0700, Luck, Tony wrote:
> On Sat, Oct 03, 2020 at 03:39:29AM +0200, Thomas Gleixner wrote:
> > On Fri, Oct 02 2020 at 13:19, Ricardo Neri wrote:
> > > Add support to discover and enumerate CPUs in Intel hybrid parts. A hybrid
> > > part has CPUs with more than one type of micro-architecture. Thus, certain
> > > features may only be present in a specific CPU type.
> > >
> > > It is useful to know the type of CPUs present in a system. For instance,
> > > perf may need to handle CPUs differently depending on the type of micro-
> > > architecture. Decoding machine check error logs may need the additional
> > > micro-architecture type information, so include that in the log.
> > 
> > 'It is useful' as justification just makes me barf.
> 
> This isn't "hetero" ... all of the cores are architecturally the same.

But it says above "A hybrid part has CPUs with more than one type of
micro-architecture."

So which is it?

> If CPUID says that some feature is supported, then it will be supported
> on all of the cores.

Ok.

> There might be some model specific performance counter events that only
> apply to some cores.

That sounds like the perf counter scheduling code would have to pay
attention to what is supported. I think we have some functionality for
that due to some AMD parts but I'd prefer if Peter comments here.

> Or a machine check error code that is logged in the model specific
> MSCOD field of IA32_MCi_STATUS. But any and all code can run on any
> core.

As long as that is consumed only by userspace I guess that's ok. The
moment someone starts to want to differentiate on what kind of CPU
kernel code runs and acts accordingly, then it becomes ugly so we better
hash it out upfront.

-- 
Regards/Gruss,
    Boris.

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