linux-kernel - [PATCH v3 Bugfix 5/6] x86/xsaves: Keep xstate_bv in init_xstate_buf header as zero for init optimimization

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Message-Id: <1431120665-36841-6-git-send-email-fenghua.yu@intel.com>
Date:	Fri,  8 May 2015 14:31:04 -0700
From:	"Fenghua Yu" <fenghua.yu@...el.com>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Dave Hansen" <dave.hansen@...el.com>,
	"Asit K Mallick" <asit.k.mallick@...el.com>,
	"Glenn Williamson" <glenn.p.williamson@...el.com>
Cc:	"linux-kernel" <linux-kernel@...r.kernel.org>,
	"x86" <x86@...nel.org>, "Fenghua Yu" <fenghua.yu@...el.com>
Subject: [PATCH v3 Bugfix 5/6] x86/xsaves: Keep xstate_bv in init_xstate_buf header as zero for init optimimization

From: Fenghua Yu <fenghua.yu@...el.com>

Keep xstate_bv in init_xstate_buf header 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 xrestor* instruction. This largely impacts context switch
performance.

Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
Reviewed-by: Dave Hansen <dave.hansen@...el.com>
---
 arch/x86/kernel/xsave.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/xsave.c b/arch/x86/kernel/xsave.c
index 4217bec..547f293 100644
--- a/arch/x86/kernel/xsave.c
+++ b/arch/x86/kernel/xsave.c
@@ -545,6 +545,12 @@ static void __init setup_init_fpu_buf(void)
 	 */
 	init_xstate_buf = alloc_bootmem_align(kernel_xstate_size,
 					      __alignof__(struct xsave_struct));
+
+	/*
+	 * Make sure xstate_bv is zero to allow init optimization work.
+	 */
+	init_xstate_buf->xsave_hdr.xstate_bv = 0;
+
 	fx_finit(&init_xstate_buf->i387);
 
 	if (!cpu_has_xsave)
@@ -552,11 +558,9 @@ static void __init setup_init_fpu_buf(void)
 
 	setup_xstate_features();
 
-	if (cpu_has_xsaves) {
+	if (cpu_has_xsaves)
 		init_xstate_buf->xsave_hdr.xcomp_bv =
 						(u64)1 << 63 | pcntxt_mask;
-		init_xstate_buf->xsave_hdr.xstate_bv = pcntxt_mask;
-	}
 
 	/*
 	 * Init all the features state with header_bv being 0x0
-- 
1.8.1.2

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