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Subject[PATCH 1/3] x86: Don't save/restore EFLAGS on task switch
Date
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@gmail.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
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