Date: Tue, 29 Nov 2005 17:17:00 +0100
From: Andi Kleen <ak@suse.de>
To: Mikael Pettersson <mikpe@csd.uu.se>
Cc: Andi Kleen <ak@suse.de>, discuss@x86-64.org, linux-kernel@vger.kernel.org
Subject: Re: Enabling RDPMC in user space by default
Message-ID: <20051129161700.GK19515@wotan.suse.de>
References: <20051129151515.GG19515@wotan.suse.de> <17292.31749.278879.822369@alkaid.it.uu.se>
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> PMC0 stops being a cycle counter as soon as any real driver
> (not the NMI watchdog) takes over the hardware, such as oprofile,
> perfmon2, or perfctr. So user-space cannot rely on the semantics

They're wrong then. oprofile shouldn't disable the NMI
watchdog. If it does it's broken and needs to be fixed.

> Disabling user-space RDTSC (setting CR4.TSD) seems evil and pointless.
> At least some users of it (the perfctr library and I hope eventually
> also perfmon2) do use it in an SMP-safe manner (through special
> user/kernel protocols).

How do you handle P state changes? I don't think it can be safely
used in user space.

-Andi
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