linux-kernel - RE: [PATCH v2 07/33] x86/intel_rdt: Add support for Cache Allocation detection

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Date:   Thu, 8 Sep 2016 13:59:02 +0000
From:   "Yu, Fenghua" <fenghua.yu@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     "Anvin, H Peter" <h.peter.anvin@...el.com>,
        Ingo Molnar <mingo@...e.hu>,
        "Luck, Tony" <tony.luck@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>, Borislav Petkov <bp@...e.de>,
        Stephane Eranian <eranian@...gle.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        "David Carrillo-Cisneros" <davidcc@...gle.com>,
        Shaohua Li <shli@...com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
        "Prakhya, Sai Praneeth" <sai.praneeth.prakhya@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: RE: [PATCH v2 07/33] x86/intel_rdt: Add support for Cache
 Allocation detection

> On Thu, 8 Sep 2016, Fenghua Yu wrote:
> > +			cpuid_count(0x00000010, 1, &eax, &ebx, &ecx,
> &edx);
> > +			c->x86_l3_max_closid = edx + 1;
> > +			c->x86_l3_max_cbm_len = eax + 1;
> 
> According to the SDM:
> 
> EAX     Bits  4:0:  Length of the capacity bit mask for the corresponding ResID.
>         Bits 31:05: Reserved
> 
> EDX	Bits 15:0:  Highest COS number supported for this ResID.
> 	Bits 31:16: Reserved
> 
> So why are we assuming that bits 31-5 of EAX and 16-31 of EDX are going to
> be zero forever and if not that they are just extending the existing bits?
> If that's the case then we don't need to mask out the upper bits, but the
> code wants a proper comment about this.

You are right. We cannot assume the upper bits are always zero. I fixed the
issue by masking out the upper bits.

Thanks.

-Fenghua