Linux context switch 64 bit

Question

in the macro switch_to in 32-bit mode, before calling the __switch_to function, the following code is executed:

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  */ 

EIP is pushed onto the stack (EIP recovery). When __switch_to ends, ret appears, which returns to this place. Here is the corresponding 64-bit code:

    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" 

Only rsp is saved and restored there. I think RIP is already at the top of the stack. But I can not find instructions where this is done. How does the 64-bit context switch really work, especially for the RIP register?

Thanks in advance!

Answer 1

In a 32-bit kernel, it thread.ipcan be one of the following:

• 1 switch_to
• ret_from_fork
• ret_from_kernel_thread

a call push + jmp.

64- thread.ip . call ( 1 32- ). , call, . ret_from_fork __switch_to ( ):

#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                                              \

ret_from_kernel_thread ret_from_fork, entry_64.S:

ENTRY(ret_from_fork)
        DEFAULT_FRAME

        LOCK ; btr $TIF_FORK,TI_flags(%r8)

        pushq_cfi $0x0002
        popfq_cfi                               # reset kernel eflags

        call schedule_tail                      # rdi: 'prev' task parameter

        GET_THREAD_INFO(%rcx)

        RESTORE_REST

        testl $3, CS-ARGOFFSET(%rsp)            # from kernel_thread?
        jz   1f