From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wi0-f177.google.com (mail-wi0-f177.google.com [209.85.212.177]) by kanga.kvack.org (Postfix) with ESMTP id EEF2D82F71 for ; Thu, 1 Oct 2015 07:02:02 -0400 (EDT) Received: by wicfx3 with SMTP id fx3so27227574wic.1 for ; Thu, 01 Oct 2015 04:02:02 -0700 (PDT) Received: from Galois.linutronix.de (Galois.linutronix.de. [2001:470:1f0b:db:abcd:42:0:1]) by mx.google.com with ESMTPS id i7si6622748wje.113.2015.10.01.04.02.01 for (version=TLSv1.2 cipher=RC4-SHA bits=128/128); Thu, 01 Oct 2015 04:02:01 -0700 (PDT) Date: Thu, 1 Oct 2015 13:01:19 +0200 (CEST) From: Thomas Gleixner Subject: Re: [PATCH 01/25] x86, fpu: add placeholder for Processor Trace XSAVE state In-Reply-To: <20150928191818.34AAC17E@viggo.jf.intel.com> Message-ID: References: <20150928191817.035A64E2@viggo.jf.intel.com> <20150928191818.34AAC17E@viggo.jf.intel.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Dave Hansen Cc: borntraeger@de.ibm.com, x86@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, dave.hansen@linux.intel.com On Mon, 28 Sep 2015, Dave Hansen wrote: > From: Dave Hansen > > There is an XSAVE state component for Intel Processor Trace. But, > we do not use it and do not expect to ever use it. > > We add a placeholder in the code for it so it is not a mystery and > also so we do not need an explicit enum initialization for Protection > Keys in a moment. > > Why will we never use it? According to Andi Kleen: > > The XSAVE support assumes that there is a single buffer > for each thread. But perf generally doesn't work this > way, it usually has only a single perf event per CPU per > user, and when tracing multiple threads on that CPU it > inherits perf event buffers between different threads. So > XSAVE per thread cannot handle this inheritance case > directly. > > Using multiple XSAVE areas (another one per perf event) > would defeat some of the state caching that the CPUs do. > > Signed-off-by: Dave Hansen Reviewed-by: Thomas Gleixner -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org