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Date: Sat, 6 Aug 2011 15:52:02 -0700
From: Andy Isaacson <adi@hexapodia.org>
To: linux-kernel@vger.kernel.org
Subject: powertop "pread cpu0 0xe8 : Bad file descriptor" fixed by
	CONFIG_MSR
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I hadn't seen this issue discussed on the list, and I don't know of a
powertop mailing list, so here's what stymied my use of powertop until I
straced it.

With CONFIG_MSR=n, powertop "1.97-2" (from Debian unstable) on 3.0.1
(and presumably many other recent kernels) exits at startup with:

pread cpu0 0xe8 : Bad file descriptor

strace shows:

open("/dev/cpu/0/msr", O_RDONLY) = -1 ENOENT (No such file or directory)
pread(4294967295, 0x7fff4c8347c8, 8, 232) = -1 EBADF (Bad file descriptor)

Setting CONFIG_MSR=m and loading msr.ko fixes the issue, allowing
powertop to work again (tested against 3.0.1).  I'm pretty sure this is
a regression from long ago -- I have had CONFIG_X86_MSR=n in almost all
configs since 2.6.34-07097-gf4b87de, and powertop worked in most of
those kernels.  But I don't have references for what versions of
powertop and what kernels I was running at various points.

-andy