arch/x86/include/asm/switch_to.h

#ifndef _ASM_X86_SWITCH_TO_H
#define _ASM_X86_SWITCH_TO_H

#include <linux/sched/task_stack.h>

struct task_struct; /* one of the stranger aspects of C forward declarations */

struct task_struct *__switch_to_asm(struct task_struct *prev,
				    struct task_struct *next);

__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);

/* This runs runs on the previous thread's stack. */
static inline void prepare_switch_to(struct task_struct *prev,
				     struct task_struct *next)
{
#ifdef CONFIG_VMAP_STACK
	/*
	 * If we switch to a stack that has a top-level paging entry
	 * that is not present in the current mm, the resulting #PF will
	 * will be promoted to a double-fault and we'll panic.  Probe
	 * the new stack now so that vmalloc_fault can fix up the page
	 * tables if needed.  This can only happen if we use a stack
	 * in vmap space.
	 *
	 * We assume that the stack is aligned so that it never spans
	 * more than one top-level paging entry.
	 *
	 * To minimize cache pollution, just follow the stack pointer.
	 */
	READ_ONCE(*(unsigned char *)next->thread.sp);
#endif
}

asmlinkage void ret_from_fork(void);

/*
 * This is the structure pointed to by thread.sp for an inactive task.  The
 * order of the fields must match the code in __switch_to_asm().
 */
struct inactive_task_frame {
#ifdef CONFIG_X86_64
	unsigned long r15;
	unsigned long r14;
	unsigned long r13;
	unsigned long r12;
#else
	unsigned long si;
	unsigned long di;
#endif
	unsigned long bx;

	/*
	 * These two fields must be together.  They form a stack frame header,
	 * needed by get_frame_pointer().
	 */
	unsigned long bp;
	unsigned long ret_addr;
};

struct fork_frame {
	struct inactive_task_frame frame;
	struct pt_regs regs;
};

#define switch_to(prev, next, last)					\
do {									\
	prepare_switch_to(prev, next);					\
									\
	((last) = __switch_to_asm((prev), (next)));			\
} while (0)

#ifdef CONFIG_X86_32
static inline void refresh_sysenter_cs(struct thread_struct *thread)
{
	/* Only happens when SEP is enabled, no need to test "SEP"arately: */
	if (unlikely(this_cpu_read(cpu_tss.x86_tss.ss1) == thread->sysenter_cs))
		return;

	this_cpu_write(cpu_tss.x86_tss.ss1, thread->sysenter_cs);
	wrmsr(MSR_IA32_SYSENTER_CS, thread->sysenter_cs, 0);
}
#endif

/* This is used when switching tasks or entering/exiting vm86 mode. */
static inline void update_sp0(struct task_struct *task)
{
#ifdef CONFIG_X86_32
	load_sp0(task->thread.sp0);
#else
	load_sp0(task_top_of_stack(task));
#endif
}

#endif /* _ASM_X86_SWITCH_TO_H */