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