CoCalc -- efi_stub

GitHub Repository: awilliam/linux-vfio
Path: blob/master/arch/x86/platform/efi/efi_stub_32.S
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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 prelog 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 prelog 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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