linux-kernel - [PATCH 1/3] x86: Don't save/restore EFLAGS on task switch

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Date:	Wed,  4 May 2016 22:44:36 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	x86@...nel.org, linux-kernel@...r.kernel.org
Cc:	Ingo Molnar <mingo@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...e.de>
Subject: [PATCH 1/3] x86: Don't save/restore EFLAGS on task switch

Now that NT is filtered by the SYSENTER entry code, it is safe to skip saving and
restoring flags on task switch.  Also remove a leftover reset of flags on 64-bit
fork.

Signed-off-by: Brian Gerst <brgerst@...il.com>
---
 arch/x86/entry/entry_32.S        | 4 ----
 arch/x86/entry/entry_64.S        | 3 ---
 arch/x86/include/asm/switch_to.h | 4 +---
 3 files changed, 1 insertion(+), 10 deletions(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 10868aa..c84d99b 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -209,8 +209,6 @@ ENTRY(ret_from_fork)
 	call	schedule_tail
 	GET_THREAD_INFO(%ebp)
 	popl	%eax
-	pushl	$0x0202				# Reset kernel eflags
-	popfl
 
 	/* When we fork, we trace the syscall return in the child, too. */
 	movl    %esp, %eax
@@ -223,8 +221,6 @@ ENTRY(ret_from_kernel_thread)
 	call	schedule_tail
 	GET_THREAD_INFO(%ebp)
 	popl	%eax
-	pushl	$0x0202				# Reset kernel eflags
-	popfl
 	movl	PT_EBP(%esp), %eax
 	call	*PT_EBX(%esp)
 	movl	$0, PT_EAX(%esp)
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 6344629..9ee0da1 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -372,9 +372,6 @@ END(ptregs_\func)
 ENTRY(ret_from_fork)
 	LOCK ; btr $TIF_FORK, TI_flags(%r8)
 
-	pushq	$0x0002
-	popfq					/* reset kernel eflags */
-
 	call	schedule_tail			/* rdi: 'prev' task parameter */
 
 	testb	$3, CS(%rsp)			/* from kernel_thread? */
diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
index 751bf4b..8f321a1 100644
--- a/arch/x86/include/asm/switch_to.h
+++ b/arch/x86/include/asm/switch_to.h
@@ -39,8 +39,7 @@ do {									\
 	 */								\
 	unsigned long ebx, ecx, edx, esi, edi;				\
 									\
-	asm volatile("pushfl\n\t"		/* save    flags */	\
-		     "pushl %%ebp\n\t"		/* save    EBP   */	\
+	asm volatile("pushl %%ebp\n\t"		/* save    EBP   */	\
 		     "movl %%esp,%[prev_sp]\n\t"	/* save    ESP   */ \
 		     "movl %[next_sp],%%esp\n\t"	/* restore ESP   */ \
 		     "movl $1f,%[prev_ip]\n\t"	/* save    EIP   */	\
@@ -49,7 +48,6 @@ do {									\
 		     "jmp __switch_to\n"	/* regparm call  */	\
 		     "1:\t"						\
 		     "popl %%ebp\n\t"		/* restore EBP   */	\
-		     "popfl\n"			/* restore flags */	\
 									\
 		     /* output parameters */				\
 		     : [prev_sp] "=m" (prev->thread.sp),		\
-- 
2.5.5