Commit b891e420 authored by Ard Biesheuvel's avatar Ard Biesheuvel
Browse files

x86/efi/mixed: Simplify and document thunking logic 



Now that the GDT/IDT and data segment selector preserve/restore logic
has been removed from the boot-time EFI mixed mode thunking routines,
the remaining logic to handle the function arguments can be simplified:
the setup of the arguments on the stack can be moved into the 32-bit
callee, which is able to use a more idiomatic sequence of PUSH
instructions.

This, in turn, allows the far call and far return to be issued using
plain LCALL and LRET instructions, removing the need to set up the
return explicitly.

Acked-by: default avatarIngo Molnar <mingo@kernel.org>
Signed-off-by: default avatarArd Biesheuvel <ardb@kernel.org>
parent 6e2da8d8

arch/x86/boot/compressed/efi_mixed.S

+37 −40
Original line number Diff line number Diff line
@@ -22,43 +22,7 @@ 
#include <asm/processor-flags.h>

#include <asm/segment.h>



	.code64

	.text

SYM_FUNC_START(__efi64_thunk)

	push	%rbp

	push	%rbx



	/* Copy args passed on stack */

	movq	0x18(%rsp), %rbp

	movq	0x20(%rsp), %rbx

	movq	0x28(%rsp), %rax



	/*

	 * Convert x86-64 ABI params to i386 ABI

	 */

	subq	$64, %rsp

	movl	%esi, 0x0(%rsp)

	movl	%edx, 0x4(%rsp)

	movl	%ecx, 0x8(%rsp)

	movl	%r8d, 0xc(%rsp)

	movl	%r9d, 0x10(%rsp)

	movl	%ebp, 0x14(%rsp)

	movl	%ebx, 0x18(%rsp)

	movl	%eax, 0x1c(%rsp)



	leaq	1f(%rip), %rbp

	movl	%cs, %ebx



	ljmpl	*efi32_call(%rip)



1:	addq	$64, %rsp

	movq	%rdi, %rax



	pop	%rbx

	pop	%rbp

	RET

SYM_FUNC_END(__efi64_thunk)



	.code32

#ifdef CONFIG_EFI_HANDOVER_PROTOCOL

SYM_FUNC_START(efi32_stub_entry)
@@ -81,11 +45,26 @@ SYM_FUNC_END(efi32_stub_entry) 
#endif



/*

 * EFI service pointer must be in %edi.

 * Called using a far call from __efi64_thunk() below, using the x86_64 SysV

 * ABI (except for R8/R9 which are inaccessible to 32-bit code - EAX/EBX are

 * used instead).  EBP+16 points to the arguments passed via the stack.

 *

 * The stack should represent the 32-bit calling convention.

 * The first argument (EDI) is a pointer to the boot service or protocol, to

 * which the remaining arguments are passed, each truncated to 32 bits.

 */

SYM_FUNC_START_LOCAL(efi_enter32)

	/*

	 * Convert x86-64 SysV ABI params to i386 ABI

	 */

	pushl	32(%ebp)	/* Up to 3 args passed via the stack */

	pushl	24(%ebp)

	pushl	16(%ebp)

	pushl	%ebx		/* R9 */

	pushl	%eax		/* R8 */

	pushl	%ecx

	pushl	%edx

	pushl	%esi



	/* Disable paging */

	movl	%cr0, %eax

	btrl	$X86_CR0_PG_BIT, %eax
@@ -104,11 +83,29 @@ SYM_FUNC_START_LOCAL(efi_enter32) 


	call	efi32_enable_long_mode



	pushl	%ebx

	pushl	%ebp

	addl	$32, %esp

	movl	%edi, %eax

	lret

SYM_FUNC_END(efi_enter32)



	.code64

SYM_FUNC_START(__efi64_thunk)

	push	%rbp

	movl	%esp, %ebp

	push	%rbx



	/* Move args #5 and #6 into 32-bit accessible registers */

	movl	%r8d, %eax

	movl	%r9d, %ebx



	lcalll	*efi32_call(%rip)



	pop	%rbx

	pop	%rbp

	RET

SYM_FUNC_END(__efi64_thunk)



	.code32

SYM_FUNC_START_LOCAL(efi32_enable_long_mode)

	movl	%cr4, %eax

	btsl	$(X86_CR4_PAE_BIT), %eax