linux-kernel - [RFC PATCH 06/11] x86,fpu: lazily skip fpu restore with eager fpu mode, too

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Date:	Sun, 11 Jan 2015 16:46:28 -0500
From:	riel@...hat.com
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...hat.com, hpa@...or.com, matt.fleming@...el.com,
	bp@...e.de, oleg@...hat.com, pbonzini@...hat.com,
	tglx@...utronix.de, luto@...capital.net
Subject: [RFC PATCH 06/11] x86,fpu: lazily skip fpu restore with eager fpu mode, too

From: Rik van Riel <riel@...hat.com>

If the next task still has its FPU state present in the FPU registers,
there is no need to restore it from memory.

This is no big deal on bare metal, where XSAVEOPT / XRSTOR are heavily
optimized, but those optimizations do not carry across VMENTER / VMEXIT.

Skipping the call to fpu_restore_checking when the FPU state is already
loaded in the CPU could save a little bit of overhead on bare metal too,
so this is not just a KVM optimization.

Signed-off-by: Rik van Riel <riel@...hat.com>
---
 arch/x86/include/asm/fpu-internal.h | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/fpu-internal.h b/arch/x86/include/asm/fpu-internal.h
index 4db8781..a5a40c7 100644
--- a/arch/x86/include/asm/fpu-internal.h
+++ b/arch/x86/include/asm/fpu-internal.h
@@ -435,13 +435,9 @@ static inline void switch_fpu_prepare(struct task_struct *old, struct task_struc
 		old->thread.fpu.last_cpu = ~0;
 		if (preload) {
 			new->thread.fpu_counter++;
-			if (!use_eager_fpu() && fpu_lazy_restore(new, cpu))
-				/* XXX: is this safe against ptrace??? */
-				__thread_fpu_begin(new);
-			else {
+			set_thread_flag(TIF_LOAD_FPU);
+			if (!fpu_lazy_restore(new, cpu))
 				prefetch(new->thread.fpu.state);
-				set_thread_flag(TIF_LOAD_FPU);
-			}
 		} else
 			/*
 			 * The new task does not want an FPU state restore,
@@ -466,6 +462,10 @@ static inline void switch_fpu_finish(void)
 
 	__thread_fpu_begin(tsk);
 
+	/* The FPU registers already have this task's FPU state. */
+	if (fpu_lazy_restore(tsk, raw_smp_processor_id()))
+		return;
+
 	if (unlikely(restore_fpu_checking(tsk)))
 		drop_init_fpu(tsk);
 }
-- 
1.9.3

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