1/* SPDX-License-Identifier: GPL-2.0 */
2/*
3 * EFI call stub for IA32.
4 *
5 * This stub allows us to make EFI calls in physical mode with interrupts
6 * turned off. Note that this implementation is different from the one in
7 * arch/x86/platform/efi/efi_stub_32.S because we're _already_ in physical
8 * mode at this point.
9 */
10
11#include <linux/linkage.h>
12#include <asm/page_types.h>
13
14/*
15 * efi_call_phys(void *, ...) is a function with variable parameters.
16 * All the callers of this function assure that all the parameters are 4-bytes.
17 */
18
19/*
20 * In gcc calling convention, EBX, ESP, EBP, ESI and EDI are all callee save.
21 * So we'd better save all of them at the beginning of this function and restore
22 * at the end no matter how many we use, because we can not assure EFI runtime
23 * service functions will comply with gcc calling convention, too.
24 */
25
26.text
27ENTRY(efi_call_phys)
28	/*
29	 * 0. The function can only be called in Linux kernel. So CS has been
30	 * set to 0x0010, DS and SS have been set to 0x0018. In EFI, I found
31	 * the values of these registers are the same. And, the corresponding
32	 * GDT entries are identical. So I will do nothing about segment reg
33	 * and GDT, but change GDT base register in prelog and epilog.
34	 */
35
36	/*
37	 * 1. Because we haven't been relocated by this point we need to
38	 * use relative addressing.
39	 */
40	call	1f
411:	popl	%edx
42	subl	$1b, %edx
43
44	/*
45	 * 2. Now on the top of stack is the return
46	 * address in the caller of efi_call_phys(), then parameter 1,
47	 * parameter 2, ..., param n. To make things easy, we save the return
48	 * address of efi_call_phys in a global variable.
49	 */
50	popl	%ecx
51	movl	%ecx, saved_return_addr(%edx)
52	/* get the function pointer into ECX*/
53	popl	%ecx
54	movl	%ecx, efi_rt_function_ptr(%edx)
55
56	/*
57	 * 3. Call the physical function.
58	 */
59	call	*%ecx
60
61	/*
62	 * 4. Balance the stack. And because EAX contain the return value,
63	 * we'd better not clobber it. We need to calculate our address
64	 * again because %ecx and %edx are not preserved across EFI function
65	 * calls.
66	 */
67	call	1f
681:	popl	%edx
69	subl	$1b, %edx
70
71	movl	efi_rt_function_ptr(%edx), %ecx
72	pushl	%ecx
73
74	/*
75	 * 10. Push the saved return address onto the stack and return.
76	 */
77	movl	saved_return_addr(%edx), %ecx
78	pushl	%ecx
79	ret
80ENDPROC(efi_call_phys)
81.previous
82
83.data
84saved_return_addr:
85	.long 0
86efi_rt_function_ptr:
87	.long 0
88