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

[Fenghua Yu]

From: Fenghua Yu <fenghua.yu@xxxxxxxxx>

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@xxxxxxxxx>
Reviewed-by: Dave Hansen <dave.hansen@xxxxxxxxx>
---
 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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