diff options
Diffstat (limited to 'arch/x86/platform/efi/efi_stub_32.S')
| -rw-r--r-- | arch/x86/platform/efi/efi_stub_32.S | 124 |
1 files changed, 124 insertions, 0 deletions
diff --git a/arch/x86/platform/efi/efi_stub_32.S b/arch/x86/platform/efi/efi_stub_32.S new file mode 100644 index 000000000..ab2e91e76 --- /dev/null +++ b/arch/x86/platform/efi/efi_stub_32.S @@ -0,0 +1,124 @@ +/* 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 +ENTRY(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 +ENDPROC(efi_call_phys) +.previous + +.data +saved_return_addr: + .long 0 +efi_rt_function_ptr: + .long 0 |