linux-kernel - [tip:x86/fpu] x86/fpu/xstate: Keep init_fpstate.xsave.header.xfeatures as zero for init optimization

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Date:	Sat, 18 Jun 2016 01:13:49 -0700
From:	tip-bot for Fenghua Yu <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	brgerst@...il.com, sai.praneeth.prakhya@...el.com, luto@...nel.org,
	tglx@...utronix.de, oleg@...hat.com, torvalds@...ux-foundation.org,
	fenghua.yu@...el.com, quentin.casasnovas@...cle.com,
	yu-cheng.yu@...el.com, dave.hansen@...el.com, hpa@...or.com,
	peterz@...radead.org, mingo@...nel.org, bp@...en8.de,
	ravi.v.shankar@...el.com, dvlasenk@...hat.com,
	linux-kernel@...r.kernel.org, dave.hansen@...ux.intel.com
Subject: [tip:x86/fpu] x86/fpu/xstate: Keep
 init_fpstate.xsave.header.xfeatures as zero for init optimization

Commit-ID:  7d9370607d28afd454775c623d5447603473a3c3
Gitweb:     http://git.kernel.org/tip/7d9370607d28afd454775c623d5447603473a3c3
Author:     Fenghua Yu <fenghua.yu@...el.com>
AuthorDate: Fri, 20 May 2016 10:47:07 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Sat, 18 Jun 2016 10:10:19 +0200

x86/fpu/xstate: Keep init_fpstate.xsave.header.xfeatures as zero for init optimization

Keep init_fpstate.xsave.header.xfeatures as zero for init optimization.
This is important for init optimization that is implemented in processor.
If a bit corresponding to an xstate in xstate_bv is 0, it means the
xstate is in init status and will not be read from memory to the processor
during XRSTOR/XRSTORS instruction. This largely impacts context switch
performance.

Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
Reviewed-by: Dave Hansen <dave.hansen@...el.com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Quentin Casasnovas <quentin.casasnovas@...cle.com>
Cc: Ravi V. Shankar <ravi.v.shankar@...el.com>
Cc: Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/2fb4ec7f18b76e8cda057a8c0038def74a9b8044.1463760376.git.yu-cheng.yu@intel.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/kernel/fpu/xstate.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 46abfaf..dbfef1b 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -329,13 +329,11 @@ static void __init setup_init_fpu_buf(void)
 	setup_xstate_features();
 	print_xstate_features();
 
-	if (boot_cpu_has(X86_FEATURE_XSAVES)) {
+	if (boot_cpu_has(X86_FEATURE_XSAVES))
 		init_fpstate.xsave.header.xcomp_bv = (u64)1 << 63 | xfeatures_mask;
-		init_fpstate.xsave.header.xfeatures = xfeatures_mask;
-	}
 
 	/*
-	 * Init all the features state with header_bv being 0x0
+	 * Init all the features state with header.xfeatures being 0x0
 	 */
 	copy_kernel_to_xregs_booting(&init_fpstate.xsave);