linux-kernel - RE: [perfmon2] comments on Performance Counters for Linux (PCL)

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Date:	Fri, 29 May 2009 09:13:10 -0700
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Andi Kleen <andi@...stfloor.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Philip Mucci <mucci@...s.utk.edu>,
	LKML <linux-kernel@...r.kernel.org>,
	Maynard Johnson <mpjohn@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...e.hu>,
	"eranian@...il.com" <eranian@...il.com>,
	perfmon2-devel <perfmon2-devel@...ts.sourceforge.net>
Subject: RE: [perfmon2] comments on Performance Counters for Linux (PCL)

>> Happen to have more details? If they really run in user context their
>> priv level would be the same right, otherwise they run in something
>> weird, not quite user not quite kernel. Now that might be the case, but
>> I'm utterly ignorant on itanic.
>
> See Documentation/ia64/fsys.txt

Summary is that the fast system calls run in ring 0 ... but since they
didn't save all the user state (in order to make them fast!) there are
restrictions on what things they can do.  In particular they must not
trap.

-Tony
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