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linux-next/arch/x86/include/asm/switch_to.h
Andy Lutomirski 2c7577a758 sched/x86_64: Don't save flags on context switch
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>
2014-10-28 11:11:30 +01:00

134 lines
4.7 KiB
C

#ifndef _ASM_X86_SWITCH_TO_H
#define _ASM_X86_SWITCH_TO_H
struct task_struct; /* one of the stranger aspects of C forward declarations */
__visible struct task_struct *__switch_to(struct task_struct *prev,
struct task_struct *next);
struct tss_struct;
void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
struct tss_struct *tss);
#ifdef CONFIG_X86_32
#ifdef CONFIG_CC_STACKPROTECTOR
#define __switch_canary \
"movl %P[task_canary](%[next]), %%ebx\n\t" \
"movl %%ebx, "__percpu_arg([stack_canary])"\n\t"
#define __switch_canary_oparam \
, [stack_canary] "=m" (stack_canary.canary)
#define __switch_canary_iparam \
, [task_canary] "i" (offsetof(struct task_struct, stack_canary))
#else /* CC_STACKPROTECTOR */
#define __switch_canary
#define __switch_canary_oparam
#define __switch_canary_iparam
#endif /* CC_STACKPROTECTOR */
/*
* Saving eflags is important. It switches not only IOPL between tasks,
* it also protects other tasks from NT leaking through sysenter etc.
*/
#define switch_to(prev, next, last) \
do { \
/* \
* Context-switching clobbers all registers, so we clobber \
* them explicitly, via unused output variables. \
* (EAX and EBP is not listed because EBP is saved/restored \
* explicitly for wchan access and EAX is the return value of \
* __switch_to()) \
*/ \
unsigned long ebx, ecx, edx, esi, edi; \
\
asm volatile("pushfl\n\t" /* save flags */ \
"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 */ \
"pushl %[next_ip]\n\t" /* restore EIP */ \
__switch_canary \
"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), \
[prev_ip] "=m" (prev->thread.ip), \
"=a" (last), \
\
/* clobbered output registers: */ \
"=b" (ebx), "=c" (ecx), "=d" (edx), \
"=S" (esi), "=D" (edi) \
\
__switch_canary_oparam \
\
/* input parameters: */ \
: [next_sp] "m" (next->thread.sp), \
[next_ip] "m" (next->thread.ip), \
\
/* regparm parameters for __switch_to(): */ \
[prev] "a" (prev), \
[next] "d" (next) \
\
__switch_canary_iparam \
\
: /* reloaded segment registers */ \
"memory"); \
} while (0)
#else /* CONFIG_X86_32 */
/* frame pointer must be last for get_wchan */
#define SAVE_CONTEXT "pushq %%rbp ; movq %%rsi,%%rbp\n\t"
#define RESTORE_CONTEXT "movq %%rbp,%%rsi ; popq %%rbp\t"
#define __EXTRA_CLOBBER \
, "rcx", "rbx", "rdx", "r8", "r9", "r10", "r11", \
"r12", "r13", "r14", "r15", "flags"
#ifdef CONFIG_CC_STACKPROTECTOR
#define __switch_canary \
"movq %P[task_canary](%%rsi),%%r8\n\t" \
"movq %%r8,"__percpu_arg([gs_canary])"\n\t"
#define __switch_canary_oparam \
, [gs_canary] "=m" (irq_stack_union.stack_canary)
#define __switch_canary_iparam \
, [task_canary] "i" (offsetof(struct task_struct, stack_canary))
#else /* CC_STACKPROTECTOR */
#define __switch_canary
#define __switch_canary_oparam
#define __switch_canary_iparam
#endif /* CC_STACKPROTECTOR */
/*
* 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) \
asm volatile(SAVE_CONTEXT \
"movq %%rsp,%P[threadrsp](%[prev])\n\t" /* save RSP */ \
"movq %P[threadrsp](%[next]),%%rsp\n\t" /* restore RSP */ \
"call __switch_to\n\t" \
"movq "__percpu_arg([current_task])",%%rsi\n\t" \
__switch_canary \
"movq %P[thread_info](%%rsi),%%r8\n\t" \
"movq %%rax,%%rdi\n\t" \
"testl %[_tif_fork],%P[ti_flags](%%r8)\n\t" \
"jnz ret_from_fork\n\t" \
RESTORE_CONTEXT \
: "=a" (last) \
__switch_canary_oparam \
: [next] "S" (next), [prev] "D" (prev), \
[threadrsp] "i" (offsetof(struct task_struct, thread.sp)), \
[ti_flags] "i" (offsetof(struct thread_info, flags)), \
[_tif_fork] "i" (_TIF_FORK), \
[thread_info] "i" (offsetof(struct task_struct, stack)), \
[current_task] "m" (current_task) \
__switch_canary_iparam \
: "memory", "cc" __EXTRA_CLOBBER)
#endif /* CONFIG_X86_32 */
#endif /* _ASM_X86_SWITCH_TO_H */