linux-kernel - Re: [PATCH 4/8] x86/intel_rdt/mba: Memory b/w allocation feature detect

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Message-ID: <20170116144105.GJ3174@twins.programming.kicks-ass.net>
Date:   Mon, 16 Jan 2017 15:41:05 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
        vikas.shivappa@...el.com, linux-kernel@...r.kernel.org,
        x86@...nel.org, hpa@...or.com, mingo@...nel.org,
        ravi.v.shankar@...el.com, tony.luck@...el.com,
        fenghua.yu@...el.com, h.peter.anvin@...el.com
Subject: Re: [PATCH 4/8] x86/intel_rdt/mba: Memory b/w allocation feature
 detect

On Mon, Jan 16, 2017 at 02:59:11PM +0100, Thomas Gleixner wrote:
> On Tue, 10 Jan 2017, Vikas Shivappa wrote:
> 
> > Detect MBA feature if CPUID.(EAX=10H, ECX=0):EBX.L2[bit 3] = 1.
> > Add supporting data structures to detect feature details which is done
> > in later patch using CPUID with EAX=10H, ECX= 3.
> 
> So why is the $subject of this patch claiming that it provides the feature
> detection?
> 
> > -/* CPUID.(EAX=10H, ECX=ResID=1).EDX */
> > +/* CPUID.(EAX=10H, ECX=ResID=3).EAX */
> > +union cpuid_0x10_3_eax {
> > +	struct {
> > +		unsigned int max_delay:12;
> > +	} split;
> 
> And the point of this struct is?

I suppose its there so we cannot forget adding it when we add more
bitfields in that word and to keep naming (full vs split) consistent wrt
other cpuid unions that do have multiple fields.

> 
> > +	unsigned int full;
> > +};