linux-kernel - Re: [PATCH 1/2] perf/x86/rapl: Add support for Intel Meteor Lake

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Date:   Fri, 6 Jan 2023 12:33:05 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     "Zhang, Rui" <rui.zhang@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "kan.liang@...ux.intel.com" <kan.liang@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] perf/x86/rapl: Add support for Intel Meteor Lake


* Borislav Petkov <bp@...en8.de> wrote:

> On Fri, Jan 06, 2023 at 11:56:18AM +0100, Ingo Molnar wrote:
> > It's a trade-off in any case: there's a point where quirk flags or even 
> > feature flags become harder to read and harder to maintain than cleanly 
> > separated per model driver functions.
> 
> Yeah, no, singular: a synthetic feature *flag*: X86_FEATURE_RAPL.
> 
> cpu/intel.c can set it and driver can test it.
> 
> Everything else inside the driver.
> 
> Until Intel can get their act together and actually do a CPUID bit like AMD. :-P
> 
> But when you think about it, whether the model matching happens in the driver or
> in cpu/intel.c doesn't matter a whole lot.
> 
> All that matters is, they should finally give it a CPUID bit.

The other thing that matters here are the RAPL *incompatibilities* between 
model variants, which are significant AFAICS.

With a CPUID we get a kind of semi-compatible hardware interface with well 
defined semantics & expansion.

With 'non-architectural', per-model RAPL features we get very little of 
that...

Which is why it's a trade-off that is hard to judge in advance: maybe we 
can simplify the code via a synthethic CPUID[s], maybe it will just be 
another zoo of per-model feature flags...

Likely won't be able to tell for sure until we see patches.

Thanks,

	Ingo