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x86/efi/mixed: Simplify and document thunking logic

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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: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
This commit is contained in:
Ard Biesheuvel
2025-01-06 14:20:35 +01:00
parent 6e2da8d87c
commit b891e4209c
1 changed files with 37 additions and 40 deletions
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77
arch/x86/boot/compressed/efi_mixed.S
View File

@@ -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