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: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Author: ardbiesheuvel

arch/x86/boot/compressed/efi_mixed.S

@@ -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