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/* SPDX-License-Identifier: GPL-2.0 */
/*
* EFI call stub for IA32.
*
* This stub allows us to make EFI calls in physical mode with interrupts
* turned off.
*/
#include <linux/linkage.h>
#include <asm/page_types.h>
/*
* efi_call_phys(void *, ...) is a function with variable parameters.
* All the callers of this function assure that all the parameters are 4-bytes.
*/
/*
* In gcc calling convention, EBX, ESP, EBP, ESI and EDI are all callee save.
* So we'd better save all of them at the beginning of this function and restore
* at the end no matter how many we use, because we can not assure EFI runtime
* service functions will comply with gcc calling convention, too.
*/
.text
SYM_FUNC_START(efi_call_phys)
/*
* 0. The function can only be called in Linux kernel. So CS has been
* set to 0x0010, DS and SS have been set to 0x0018. In EFI, I found
* the values of these registers are the same. And, the corresponding
* GDT entries are identical. So I will do nothing about segment reg
* and GDT, but change GDT base register in prolog and epilog.
*/
/*
* 1. Now I am running with EIP = <physical address> + PAGE_OFFSET.
* But to make it smoothly switch from virtual mode to flat mode.
* The mapping of lower virtual memory has been created in prolog and
* epilog.
*/
movl $1f, %edx
subl $__PAGE_OFFSET, %edx
jmp *%edx
1:
/*
* 2. Now on the top of stack is the return
* address in the caller of efi_call_phys(), then parameter 1,
* parameter 2, ..., param n. To make things easy, we save the return
* address of efi_call_phys in a global variable.
*/
popl %edx
movl %edx, saved_return_addr
/* get the function pointer into ECX*/
popl %ecx
movl %ecx, efi_rt_function_ptr
movl $2f, %edx
subl $__PAGE_OFFSET, %edx
pushl %edx
/*
* 3. Clear PG bit in %CR0.
*/
movl %cr0, %edx
andl $0x7fffffff, %edx
movl %edx, %cr0
jmp 1f
1:
/*
* 4. Adjust stack pointer.
*/
subl $__PAGE_OFFSET, %esp
/*
* 5. Call the physical function.
*/
jmp *%ecx
2:
/*
* 6. After EFI runtime service returns, control will return to
* following instruction. We'd better readjust stack pointer first.
*/
addl $__PAGE_OFFSET, %esp
/*
* 7. Restore PG bit
*/
movl %cr0, %edx
orl $0x80000000, %edx
movl %edx, %cr0
jmp 1f
1:
/*
* 8. Now restore the virtual mode from flat mode by
* adding EIP with PAGE_OFFSET.
*/
movl $1f, %edx
jmp *%edx
1:
/*
* 9. Balance the stack. And because EAX contain the return value,
* we'd better not clobber it.
*/
leal efi_rt_function_ptr, %edx
movl (%edx), %ecx
pushl %ecx
/*
* 10. Push the saved return address onto the stack and return.
*/
leal saved_return_addr, %edx
movl (%edx), %ecx
pushl %ecx
ret
SYM_FUNC_END(efi_call_phys)
.previous
.data
saved_return_addr:
.long 0
efi_rt_function_ptr:
.long 0
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