x86/boot/compressed: Move bootargs parsing out of 32-bit startup code

Move the logic that chooses between the different EFI entrypoints out of
the 32-bit boot path, and into a 64-bit helper that can perform the same
task much more cleanly. While at it, document the mixed mode boot flow
in a code comment.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20221122161017.2426828-4-ardb@kernel.org

arch/x86/boot/compressed/efi_mixed.S

@@ -22,6 +22,49 @@
 
 	.code64
 	.text
+/*
+ * When booting in 64-bit mode on 32-bit EFI firmware, startup_64_mixed_mode()
+ * is the first thing that runs after switching to long mode. Depending on
+ * whether the EFI handover protocol or the compat entry point was used to
+ * enter the kernel, it will either branch to the 64-bit EFI handover
+ * entrypoint at offset 0x390 in the image, or to the 64-bit EFI PE/COFF
+ * entrypoint efi_pe_entry(). In the former case, the bootloader must provide a
+ * struct bootparams pointer as the third argument, so the presence of such a
+ * pointer is used to disambiguate.
+ *
+ *                                                             +--------------+
+ *  +------------------+     +------------+            +------>| efi_pe_entry |
+ *  | efi32_pe_entry   |---->|            |            |       +-----------+--+
+ *  +------------------+     |            |     +------+----------------+  |
+ *                           | startup_32 |---->| startup_64_mixed_mode |  |
+ *  +------------------+     |            |     +------+----------------+  V
+ *  | efi32_stub_entry |---->|            |            |     +------------------+
+ *  +------------------+     +------------+            +---->| efi64_stub_entry |
+ *                                                           +-------------+----+
+ *                           +------------+     +----------+               |
+ *                           | startup_64 |<----| efi_main |<--------------+
+ *                           +------------+     +----------+
+ */
+SYM_FUNC_START(startup_64_mixed_mode)
+	lea	efi32_boot_args(%rip), %rdx
+	mov	0(%rdx), %edi
+	mov	4(%rdx), %esi
+	mov	8(%rdx), %edx		// saved bootparams pointer
+	test	%edx, %edx
+	jnz	efi64_stub_entry
+	/*
+	 * efi_pe_entry uses MS calling convention, which requires 32 bytes of
+	 * shadow space on the stack even if all arguments are passed in
+	 * registers. We also need an additional 8 bytes for the space that
+	 * would be occupied by the return address, and this also results in
+	 * the correct stack alignment for entry.
+	 */
+	sub	$40, %rsp
+	mov	%rdi, %rcx		// MS calling convention
+	mov	%rsi, %rdx
+	jmp	efi_pe_entry
+SYM_FUNC_END(startup_64_mixed_mode)
+
 SYM_FUNC_START(__efi64_thunk)
 	push	%rbp
 	push	%rbx

arch/x86/boot/compressed/head_64.S

@@ -261,25 +261,9 @@ SYM_FUNC_START(startup_32)
 	 */
 	leal	rva(startup_64)(%ebp), %eax
 #ifdef CONFIG_EFI_MIXED
-	movl	rva(efi32_boot_args)(%ebp), %edi
-	testl	%edi, %edi
-	jz	1f
-	leal	rva(efi64_stub_entry)(%ebp), %eax
-	movl	rva(efi32_boot_args+4)(%ebp), %esi
-	movl	rva(efi32_boot_args+8)(%ebp), %edx	// saved bootparams pointer
-	testl	%edx, %edx
-	jnz	1f
-	/*
-	 * efi_pe_entry uses MS calling convention, which requires 32 bytes of
-	 * shadow space on the stack even if all arguments are passed in
-	 * registers. We also need an additional 8 bytes for the space that
-	 * would be occupied by the return address, and this also results in
-	 * the correct stack alignment for entry.
-	 */
-	subl	$40, %esp
-	leal	rva(efi_pe_entry)(%ebp), %eax
-	movl	%edi, %ecx			// MS calling convention
-	movl	%esi, %edx
+	cmpb	$1, rva(efi_is64)(%ebp)
+	je	1f
+	leal	rva(startup_64_mixed_mode)(%ebp), %eax
 1:
 #endif
 	/* Check if the C-bit position is correct when SEV is active */
@@ -779,7 +763,7 @@ SYM_DATA_END_LABEL(boot32_idt, SYM_L_GLOBAL, boot32_idt_end)
 SYM_DATA(image_offset, .long 0)
 #endif
 #ifdef CONFIG_EFI_MIXED
-SYM_DATA_LOCAL(efi32_boot_args, .long 0, 0, 0)
+SYM_DATA(efi32_boot_args, .long 0, 0, 0)
 SYM_DATA(efi_is64, .byte 1)
 
 #define ST32_boottime		60 // offsetof(efi_system_table_32_t, boottime)