root/tools/testing/selftests/x86/thunks.S

   1 /* SPDX-License-Identifier: GPL-2.0-only */
   2 /*
   3  * thunks.S - assembly helpers for mixed-bitness code
   4  * Copyright (c) 2015 Andrew Lutomirski
   5  *
   6  * These are little helpers that make it easier to switch bitness on
   7  * the fly.
   8  */
   9 
  10         .text
  11 
  12         .global call32_from_64
  13         .type call32_from_64, @function
  14 call32_from_64:
  15         // rdi: stack to use
  16         // esi: function to call
  17 
  18         // Save registers
  19         pushq %rbx
  20         pushq %rbp
  21         pushq %r12
  22         pushq %r13
  23         pushq %r14
  24         pushq %r15
  25         pushfq
  26 
  27         // Switch stacks
  28         mov %rsp,(%rdi)
  29         mov %rdi,%rsp
  30 
  31         // Switch to compatibility mode
  32         pushq $0x23  /* USER32_CS */
  33         pushq $1f
  34         lretq
  35 
  36 1:
  37         .code32
  38         // Call the function
  39         call *%esi
  40         // Switch back to long mode
  41         jmp $0x33,$1f
  42         .code64
  43 
  44 1:
  45         // Restore the stack
  46         mov (%rsp),%rsp
  47 
  48         // Restore registers
  49         popfq
  50         popq %r15
  51         popq %r14
  52         popq %r13
  53         popq %r12
  54         popq %rbp
  55         popq %rbx
  56 
  57         ret
  58 
  59 .size call32_from_64, .-call32_from_64