Date: Wed, 30 Nov 2005 00:39:20 +0100
From: Andi Kleen <ak@suse.de>
To: Nicholas Miell <nmiell@comcast.net>
Cc: Andi Kleen <ak@suse.de>, Stephane Eranian <eranian@hpl.hp.com>,
       Ray Bryant <raybry@mpdtxmail.amd.com>, discuss@x86-64.org,
       linux-kernel@vger.kernel.org, perfctr-devel@lists.sourceforge.net
Subject: Re: [Perfctr-devel] Re: Enabling RDPMC in user space by default
Message-ID: <20051129233920.GW19515@wotan.suse.de>
References: <200511291056.32455.raybry@mpdtxmail.amd.com> <20051129180903.GB6611@frankl.hpl.hp.com> <20051129181344.GN19515@wotan.suse.de> <1133300591.3271.1.camel@entropy> <20051129215207.GR19515@wotan.suse.de> <1133303615.3271.12.camel@entropy> <20051129224346.GS19515@wotan.suse.de> <1133305338.3271.30.camel@entropy> <20051129231750.GU19515@wotan.suse.de> <1133306966.3271.36.camel@entropy>
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> Well, if that's all you want them to use RDPMC 0 for, why not just make
> PMCs programmable from userspace?

First we need to have a cycle counter PMC anyways for the NMI watchdog.
So it can as well be used for other purposes.

And using virtual performance counters adds a large cost the 
context switch for changing the MSRs around. An always running counter
avoids this problem.

-Andi
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