x86/efi: Firmware agnostic handover entry points

The EFI handover code only works if the "bitness" of the firmware and
the kernel match, i.e. 64-bit firmware and 64-bit kernel - it is not
possible to mix the two. This goes against the tradition that a 32-bit
kernel can be loaded on a 64-bit BIOS platform without having to do
anything special in the boot loader. Linux distributions, for one thing,
regularly run only 32-bit kernels on their live media.

Despite having only one 'handover_offset' field in the kernel header,
EFI boot loaders use two separate entry points to enter the kernel based
on the architecture the boot loader was compiled for,

    (1) 32-bit loader: handover_offset
    (2) 64-bit loader: handover_offset + 512

Since we already have two entry points, we can leverage them to infer
the bitness of the firmware we're running on, without requiring any boot
loader modifications, by making (1) and (2) valid entry points for both
CONFIG_X86_32 and CONFIG_X86_64 kernels.

To be clear, a 32-bit boot loader will always use (1) and a 64-bit boot
loader will always use (2). It's just that, if a single kernel image
supports (1) and (2) that image can be used with both 32-bit and 64-bit
boot loaders, and hence both 32-bit and 64-bit EFI.

(1) and (2) must be 512 bytes apart at all times, but that is already
part of the boot ABI and we could never change that delta without
breaking existing boot loaders anyhow.

Signed-off-by: Matt Fleming <matt.fleming@intel.com>

arch/x86/boot/compressed/head_64.S

@@ -178,6 +178,13 @@ ENTRY(startup_32)
 	 */
 	pushl	$__KERNEL_CS
 	leal	startup_64(%ebp), %eax
+#ifdef CONFIG_EFI_MIXED
+	movl	efi32_config(%ebp), %ebx
+	cmp	$0, %ebx
+	jz	1f
+	leal	handover_entry(%ebp), %eax
+1:
+#endif
 	pushl	%eax
 
 	/* Enter paged protected Mode, activating Long Mode */
@@ -188,6 +195,30 @@ ENTRY(startup_32)
 	lret
 ENDPROC(startup_32)
 
+#ifdef CONFIG_EFI_MIXED
+	.org 0x190
+ENTRY(efi32_stub_entry)
+	add	$0x4, %esp		/* Discard return address */
+	popl	%ecx
+	popl	%edx
+	popl	%esi
+
+	leal	(BP_scratch+4)(%esi), %esp
+	call	1f
+1:	pop	%ebp
+	subl	$1b, %ebp
+
+	movl	%ecx, efi32_config(%ebp)
+	movl	%edx, efi32_config+8(%ebp)
+	sgdtl	efi32_boot_gdt(%ebp)
+
+	leal	efi32_config(%ebp), %eax
+	movl	%eax, efi_config(%ebp)
+
+	jmp	startup_32
+ENDPROC(efi32_stub_entry)
+#endif
+
 	.code64
 	.org 0x200
 ENTRY(startup_64)
@@ -231,13 +262,7 @@ ENTRY(efi_pe_entry)
 	mov	%rax, %rsi
 	jmp	2f		/* Skip the relocation */
 
-ENTRY(efi_stub_entry)
-	movq	%rdi, efi64_config(%rip)	/* Handle */
-	movq	%rsi, efi64_config+8(%rip) /* EFI System table pointer */
-
-	leaq	efi64_config(%rip), %rax
-	movq	%rax, efi_config(%rip)
-
+handover_entry:
 	call	1f
 1:	popq	%rbp
 	subq	$1b, %rbp
@@ -247,7 +272,6 @@ ENTRY(efi_stub_entry)
 	 */
 	movq	efi_config(%rip), %rax
 	addq	%rbp, 88(%rax)
-	movq	%rdx, %rsi
 2:
 	movq	efi_config(%rip), %rdi
 	call	efi_main
@@ -336,6 +360,20 @@ preferred_addr:
 	leaq	relocated(%rbx), %rax
 	jmp	*%rax
 
+#ifdef CONFIG_EFI_STUB
+	.org 0x390
+ENTRY(efi64_stub_entry)
+	movq	%rdi, efi64_config(%rip)	/* Handle */
+	movq	%rsi, efi64_config+8(%rip) /* EFI System table pointer */
+
+	leaq	efi64_config(%rip), %rax
+	movq	%rax, efi_config(%rip)
+
+	movq	%rdx, %rsi
+	jmp	handover_entry
+ENDPROC(efi64_stub_entry)
+#endif
+
 	.text
 relocated:
 
@@ -404,6 +442,14 @@ gdt_end:
 efi_config:
 	.quad	0
 
+#ifdef CONFIG_EFI_MIXED
+	.global efi32_config
+efi32_config:
+	.fill	11,8,0
+	.quad	efi64_thunk
+	.byte	0
+#endif
+
 	.global efi64_config
 efi64_config:
 	.fill	11,8,0