/* 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. Note that this implementation is different from the one in
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* arch/x86/platform/efi/efi_stub_32.S because we're _already_ in physical
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* mode at this point.
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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 prelog and epilog.
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*/
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/*
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* 1. Because we haven't been relocated by this point we need to
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* use relative addressing.
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*/
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call 1f
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1: popl %edx
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subl $1b, %edx
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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 %ecx
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movl %ecx, saved_return_addr(%edx)
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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(%edx)
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/*
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* 3. Call the physical function.
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*/
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call *%ecx
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/*
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* 4. Balance the stack. And because EAX contain the return value,
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* we'd better not clobber it. We need to calculate our address
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* again because %ecx and %edx are not preserved across EFI function
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* calls.
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*/
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call 1f
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1: popl %edx
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subl $1b, %edx
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movl efi_rt_function_ptr(%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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movl saved_return_addr(%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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