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sched/x86_64: Don't save flags on context switch

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Now that the kernel always runs with clean flags (in particular,
NT is clear), there is no need to save and restore flags on
every context switch.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Cc: Sebastian Lackner <sebastian@fds-team.de>
Cc: Anish Bhatt <anish@chelsio.com>
Cc: Chuck Ebbert <cebbert.lkml@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jan Beulich <jbeulich@suse.com>
Link: http://lkml.kernel.org/r/bf6fb790787eb95b922157838f52712c25dda157.1412187233.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Andy Lutomirski 2014-10-01 11:28:25 -07:00 committed by Ingo Molnar
parent cac7f24298
commit 2c7577a758
1 changed files with 8 additions and 4 deletions
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12
arch/x86/include/asm/switch_to.h
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@ -79,12 +79,12 @@ do { \
#else /* CONFIG_X86_32 */ #else /* CONFIG_X86_32 */
/* frame pointer must be last for get_wchan */ /* frame pointer must be last for get_wchan */
#define SAVE_CONTEXT "pushf ; pushq %%rbp ; movq %%rsi,%%rbp\n\t" #define SAVE_CONTEXT "pushq %%rbp ; movq %%rsi,%%rbp\n\t"
#define RESTORE_CONTEXT "movq %%rbp,%%rsi ; popq %%rbp ; popf\t" #define RESTORE_CONTEXT "movq %%rbp,%%rsi ; popq %%rbp\t"
#define __EXTRA_CLOBBER \ #define __EXTRA_CLOBBER \
, "rcx", "rbx", "rdx", "r8", "r9", "r10", "r11", \ , "rcx", "rbx", "rdx", "r8", "r9", "r10", "r11", \
"r12", "r13", "r14", "r15" "r12", "r13", "r14", "r15", "flags"
#ifdef CONFIG_CC_STACKPROTECTOR #ifdef CONFIG_CC_STACKPROTECTOR
#define __switch_canary \ #define __switch_canary \
@ -100,7 +100,11 @@ do { \
#define __switch_canary_iparam #define __switch_canary_iparam
#endif /* CC_STACKPROTECTOR */ #endif /* CC_STACKPROTECTOR */
/* Save restore flags to clear handle leaking NT */ /*
* There is no need to save or restore flags, because flags are always
* clean in kernel mode, with the possible exception of IOPL. Kernel IOPL
* has no effect.
*/
#define switch_to(prev, next, last) \ #define switch_to(prev, next, last) \
asm volatile(SAVE_CONTEXT \ asm volatile(SAVE_CONTEXT \
"movq %%rsp,%P[threadrsp](%[prev])\n\t" /* save RSP */ \ "movq %%rsp,%P[threadrsp](%[prev])\n\t" /* save RSP */ \