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Date: Fri, 4 Dec 2015 23:14:46 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>,
        Amy Wiles <amy.l.wiles@intel.com>,
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        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] x86/rapl: Do not load in a guest
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On Fri, Dec 04, 2015 at 09:51:02AM -0800, Jacob Pan wrote:
> On Fri, 4 Dec 2015 09:22:56 +0100
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > Also, yuck @ powercap/intel_rapl.c for doing rdmsr_on_cpu() +
> > wrmsr_on_cpu() all over the place.
> Can you please be more specific? is the concern related to the
> overhead of IPI? I am doing these calls based on MSR CPU scope and
> consider the fact that access is less frequent.

Yeah, its just offensive to do an IPI to read a value, then twiddle a
few bits on the value and then IPI again to store the value.

I know its low freq, and that MSR access is slow, but *groan*.