linux-kernel - [PATCH 01/26] x86, fpu: add placeholder for Processor Trace XSAVE state

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Message-Id: <20150916174904.E002DB09@viggo.jf.intel.com>
Date:	Wed, 16 Sep 2015 10:49:04 -0700
From:	Dave Hansen <dave@...1.net>
To:	dave@...1.net
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH 01/26] x86, fpu: add placeholder for Processor Trace XSAVE state

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.

---

 b/arch/x86/include/asm/fpu/types.h |    1 +
 b/arch/x86/kernel/fpu/xstate.c     |    3 ++-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff -puN arch/x86/include/asm/fpu/types.h~pt-xstate-bit arch/x86/include/asm/fpu/types.h
--- a/arch/x86/include/asm/fpu/types.h~pt-xstate-bit	2015-09-16 10:48:11.570979927 -0700
+++ b/arch/x86/include/asm/fpu/types.h	2015-09-16 10:48:11.574980109 -0700
@@ -108,6 +108,7 @@ enum xfeature {
 	XFEATURE_OPMASK,
 	XFEATURE_ZMM_Hi256,
 	XFEATURE_Hi16_ZMM,
+	XFEATURE_PT_UNIMPLEMENTED_SO_FAR,

 	XFEATURE_MAX,
 };
diff -puN arch/x86/kernel/fpu/xstate.c~pt-xstate-bit arch/x86/kernel/fpu/xstate.c
--- a/arch/x86/kernel/fpu/xstate.c~pt-xstate-bit	2015-09-16 10:48:11.571979973 -0700
+++ b/arch/x86/kernel/fpu/xstate.c	2015-09-16 10:48:11.575980154 -0700
@@ -469,7 +469,8 @@ static void check_xstate_against_struct(
 	 * numbers.
 	 */
 	if ((nr < XFEATURE_YMM) ||
-	    (nr >= XFEATURE_MAX)) {
+	    (nr >= XFEATURE_MAX) ||
+	    (nr == XFEATURE_PT_UNIMPLEMENTED_SO_FAR)) {
 		WARN_ONCE(1, "no structure for xstate: %d\n", nr);
 		XSTATE_WARN_ON(1);
 	}
_
--
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