x86/efi/mixed: Remove dependency on legacy startup_32 code

The EFI mixed mode startup code calls into startup_32 in the legacy
decompressor with a mocked up boot_params struct, only to get it to set
up the 1:1 mapping of the lower 4 GiB of memory and switch to a GDT that
supports 64-bit mode.

In order to be able to reuse the EFI mixed mode startup code in EFI
zboot images, which do not incorporate the legacy decompressor code,
decouple it, by dealing with the GDT and IDT directly.

Doing so makes it possible to construct a GDT that is compatible with
the one the firmware uses, with one additional entry for a 64-bit mode
code segment appended. This removes the need entirely to switch between
GDTs and IDTs or data segment selector values and all of this code can
be removed.

Acked-by: Ingo Molnar <[email protected]>
Signed-off-by: Ard Biesheuvel <[email protected]>

Author: ardbiesheuvel

arch/x86/boot/compressed/efi_mixed.S

@@ -15,70 +15,23 @@
  */
 
 #include <linux/linkage.h>
-#include <asm/asm-offsets.h>
+#include <asm/desc_defs.h>
 #include <asm/msr.h>
 #include <asm/page_types.h>
 #include <asm/pgtable_types.h>
 #include <asm/processor-flags.h>
 #include <asm/segment.h>
-#include <asm/setup.h>
 
 	.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 common 64-bit EFI stub
- * entrypoint efi_stub_entry() directly, or via 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 |  |
- *  +------------------+     |            |     +------+----------------+  |
- *  | efi32_stub_entry |---->|            |            |                   |
- *  +------------------+     +------------+            |                   |
- *                                                     V                   |
- *                           +------------+     +----------------+         |
- *                           | startup_64 |<----| efi_stub_entry |<--------+
- *                           +------------+     +----------------+
- */
-SYM_FUNC_START(startup_64_mixed_mode)
-	lea	efi32_boot_args(%rip), %rdx
-	mov	0(%rdx), %edi
-	mov	4(%rdx), %esi
-
-	leaq	(pte + 5 * PAGE_SIZE)(%rip), %rax
-	movq	%rax, %cr3		// reload after startup_32
-
-	/* Switch to the firmware's stack */
-	movl	efi32_boot_sp(%rip), %esp
-	andl	$~7, %esp
-
-	mov	8(%rdx), %edx		// saved bootparams pointer
-	call	efi_stub_entry
-SYM_FUNC_END(startup_64_mixed_mode)
-
 SYM_FUNC_START(__efi64_thunk)
 	push	%rbp
 	push	%rbx
 
-	movl	%ds, %eax
-	push	%rax
-	movl	%es, %eax
-	push	%rax
-	movl	%ss, %eax
-	push	%rax
-
 	/* Copy args passed on stack */
-	movq	0x30(%rsp), %rbp
-	movq	0x38(%rsp), %rbx
-	movq	0x40(%rsp), %rax
+	movq	0x18(%rsp), %rbp
+	movq	0x20(%rsp), %rbx
+	movq	0x28(%rsp), %rax
 
 	/*
 	 * Convert x86-64 ABI params to i386 ABI
@@ -93,44 +46,14 @@ SYM_FUNC_START(__efi64_thunk)
 	movl	%ebx, 0x18(%rsp)
 	movl	%eax, 0x1c(%rsp)
 
-	leaq	0x20(%rsp), %rbx
-	sgdt	(%rbx)
-	sidt	16(%rbx)
-
 	leaq	1f(%rip), %rbp
+	movl	%cs, %ebx
 
-	/*
-	 * Switch to IDT and GDT with 32-bit segments. These are the firmware
-	 * GDT and IDT that were installed when the kernel started executing.
-	 * The pointers were saved by the efi32_entry() routine below.
-	 *
-	 * Pass the saved DS selector to the 32-bit code, and use far return to
-	 * restore the saved CS selector.
-	 */
-	lidt	efi32_boot_idt(%rip)
-	lgdt	efi32_boot_gdt(%rip)
-
-	movzwl	efi32_boot_ds(%rip), %edx
-	movzwq	efi32_boot_cs(%rip), %rax
-	pushq	%rax
-	leaq	efi_enter32(%rip), %rax
-	pushq	%rax
-	lretq
+	ljmpl	*efi32_call(%rip)
 
 1:	addq	$64, %rsp
 	movq	%rdi, %rax
 
-	pop	%rbx
-	movl	%ebx, %ss
-	pop	%rbx
-	movl	%ebx, %es
-	pop	%rbx
-	movl	%ebx, %ds
-	/* Clear out 32-bit selector from FS and GS */
-	xorl	%ebx, %ebx
-	movl	%ebx, %fs
-	movl	%ebx, %gs
-
 	pop	%rbx
 	pop	%rbp
 	RET
@@ -141,7 +64,6 @@ SYM_FUNC_END(__efi64_thunk)
 SYM_FUNC_START(efi32_stub_entry)
 	call	1f
 1:	popl	%ecx
-	leal	(efi32_boot_args - 1b)(%ecx), %ebx
 
 	/* Clear BSS */
 	xorl	%eax, %eax
@@ -153,11 +75,8 @@ SYM_FUNC_START(efi32_stub_entry)
 	rep	stosl
 
 	add	$0x4, %esp		/* Discard return address */
-	popl	%ecx
-	popl	%edx
-	popl	%esi
-	movl	%esi, 8(%ebx)
-	jmp	efi32_entry
+	movl	8(%esp), %ebx		/* struct boot_params pointer */
+	jmp	efi32_startup
 SYM_FUNC_END(efi32_stub_entry)
 #endif
 
@@ -167,13 +86,6 @@ SYM_FUNC_END(efi32_stub_entry)
  * The stack should represent the 32-bit calling convention.
  */
 SYM_FUNC_START_LOCAL(efi_enter32)
-	/* Load firmware selector into data and stack segment registers */
-	movl	%edx, %ds
-	movl	%edx, %es
-	movl	%edx, %fs
-	movl	%edx, %gs
-	movl	%edx, %ss
-
 	/* Disable paging */
 	movl	%cr0, %eax
 	btrl	$X86_CR0_PG_BIT, %eax
@@ -190,21 +102,9 @@ SYM_FUNC_START_LOCAL(efi_enter32)
 	/* We must preserve return value */
 	movl	%eax, %edi
 
-	/*
-	 * Some firmware will return with interrupts enabled. Be sure to
-	 * disable them before we switch GDTs and IDTs.
-	 */
-	cli
-
-	lidtl	16(%ebx)
-	lgdtl	(%ebx)
-
-	xorl	%eax, %eax
-	lldt	%ax
-
 	call	efi32_enable_long_mode
 
-	pushl	$__KERNEL_CS
+	pushl	%ebx
 	pushl	%ebp
 	lret
 SYM_FUNC_END(efi_enter32)
@@ -230,50 +130,56 @@ SYM_FUNC_START_LOCAL(efi32_enable_long_mode)
 SYM_FUNC_END(efi32_enable_long_mode)
 
 /*
- * This is the common EFI stub entry point for mixed mode.
+ * This is the common EFI stub entry point for mixed mode. It sets up the GDT
+ * and page tables needed for 64-bit execution, after which it calls the
+ * common 64-bit EFI entrypoint efi_stub_entry().
  *
- * Arguments:	%ecx	image handle
- * 		%edx	EFI system table pointer
+ * Arguments:	0(%esp)	image handle
+ * 		4(%esp)	EFI system table pointer
+ *		%ebx	struct boot_params pointer (or NULL)
  *
  * Since this is the point of no return for ordinary execution, no registers
  * are considered live except for the function parameters. [Note that the EFI
  * stub may still exit and return to the firmware using the Exit() EFI boot
  * service.]
  */
-SYM_FUNC_START_LOCAL(efi32_entry)
-	call	1f
-1:	pop	%ebx
-
-	/* Save firmware GDTR and code/data selectors */
-	sgdtl	(efi32_boot_gdt - 1b)(%ebx)
-	movw	%cs, (efi32_boot_cs - 1b)(%ebx)
-	movw	%ds, (efi32_boot_ds - 1b)(%ebx)
-
-	/* Store firmware IDT descriptor */
-	sidtl	(efi32_boot_idt - 1b)(%ebx)
-
-	/* Store firmware stack pointer */
-	movl	%esp, (efi32_boot_sp - 1b)(%ebx)
-
-	/* Store boot arguments */
-	leal	(efi32_boot_args - 1b)(%ebx), %ebx
-	movl	%ecx, 0(%ebx)
-	movl	%edx, 4(%ebx)
-	movb	$0x0, 12(%ebx)          // efi_is64
-
-	/*
-	 * Allocate some memory for a temporary struct boot_params, which only
-	 * needs the minimal pieces that startup_32() relies on.
-	 */
-	subl	$PARAM_SIZE, %esp
-	movl	%esp, %esi
-	movl	$PAGE_SIZE, BP_kernel_alignment(%esi)
-	movl	$_end - 1b, BP_init_size(%esi)
-	subl	$startup_32 - 1b, BP_init_size(%esi)
+SYM_FUNC_START_LOCAL(efi32_startup)
+	movl	%esp, %ebp
+
+	subl	$8, %esp
+	sgdtl	(%esp)			/* Save GDT descriptor to the stack */
+	movl	2(%esp), %esi		/* Existing GDT pointer */
+	movzwl	(%esp), %ecx		/* Existing GDT limit */
+	inc	%ecx			/* Existing GDT size */
+	andl	$~7, %ecx		/* Ensure size is multiple of 8 */
+
+	subl	%ecx, %esp		/* Allocate new GDT */
+	andl	$~15, %esp		/* Realign the stack */
+	movl	%esp, %edi		/* New GDT address */
+	leal	7(%ecx), %eax		/* New GDT limit */
+	pushw	%cx			/* Push 64-bit CS (for LJMP below) */
+	pushl	%edi			/* Push new GDT address */
+	pushw	%ax			/* Push new GDT limit */
+
+	/* Copy GDT to the stack and add a 64-bit code segment at the end */
+	movl	$GDT_ENTRY(DESC_CODE64, 0, 0xfffff) & 0xffffffff, (%edi,%ecx)
+	movl	$GDT_ENTRY(DESC_CODE64, 0, 0xfffff) >> 32, 4(%edi,%ecx)
+	shrl	$2, %ecx
+	cld
+	rep	movsl			/* Copy the firmware GDT */
+	lgdtl	(%esp)			/* Switch to the new GDT */
 
 	call	1f
 1:	pop	%edi
 
+	/* Record mixed mode entry */
+	movb	$0x0, (efi_is64 - 1b)(%edi)
+
+	/* Set up indirect far call to re-enter 32-bit mode */
+	leal	(efi32_call - 1b)(%edi), %eax
+	addl	%eax, (%eax)
+	movw	%cs, 4(%eax)
+
 	/* Disable paging */
 	movl	%cr0, %eax
 	btrl	$X86_CR0_PG_BIT, %eax
@@ -297,8 +203,17 @@ SYM_FUNC_START_LOCAL(efi32_entry)
 	movl	%edx, (%eax)
 	movl	%eax, %cr3
 
-	jmp	startup_32
-SYM_FUNC_END(efi32_entry)
+	call	efi32_enable_long_mode
+
+	/* Set up far jump to 64-bit mode (CS is already on the stack) */
+	leal	(efi_stub_entry - 1b)(%edi), %eax
+	movl	%eax, 2(%esp)
+
+	movl	0(%ebp), %edi
+	movl	4(%ebp), %esi
+	movl	%ebx, %edx
+	ljmpl	*2(%esp)
+SYM_FUNC_END(efi32_startup)
 
 /*
  * efi_status_t efi32_pe_entry(efi_handle_t image_handle,
@@ -313,10 +228,8 @@ SYM_FUNC_START(efi32_pe_entry)
 	btl	$29, %edx			// check long mode bit
 	jnc	1f
 	leal	8(%esp), %esp			// preserve stack alignment
-	movl	(%esp), %ecx			// image_handle
-	movl	4(%esp), %edx			// sys_table
-	jmp	efi32_entry			// pass %ecx, %edx
-						// no other registers remain live
+	xor	%ebx, %ebx			// no struct boot_params pointer
+	jmp	efi32_startup			// only ESP and EBX remain live
 1:	movl	$0x80000003, %eax		// EFI_UNSUPPORTED
 	popl	%ebx
 	RET
@@ -332,20 +245,10 @@ SYM_FUNC_END(efi64_stub_entry)
 
 	.data
 	.balign	8
-SYM_DATA_START_LOCAL(efi32_boot_gdt)
-	.word	0
-	.quad	0
-SYM_DATA_END(efi32_boot_gdt)
-
-SYM_DATA_START_LOCAL(efi32_boot_idt)
-	.word	0
-	.quad	0
-SYM_DATA_END(efi32_boot_idt)
-
-SYM_DATA_LOCAL(efi32_boot_cs, .word 0)
-SYM_DATA_LOCAL(efi32_boot_ds, .word 0)
-SYM_DATA_LOCAL(efi32_boot_sp, .long 0)
-SYM_DATA_LOCAL(efi32_boot_args, .long 0, 0, 0)
+SYM_DATA_START_LOCAL(efi32_call)
+	.long	efi_enter32 - .
+	.word	0x0
+SYM_DATA_END(efi32_call)
 SYM_DATA(efi_is64, .byte 1)
 
 	.bss

arch/x86/boot/compressed/head_64.S

@@ -263,13 +263,6 @@ SYM_FUNC_START(startup_32)
 	 * used to perform that far jump.
 	 */
 	leal	rva(startup_64)(%ebp), %eax
-#ifdef CONFIG_EFI_MIXED
-	cmpb	$1, rva(efi_is64)(%ebp)
-	je	1f
-	leal	rva(startup_64_mixed_mode)(%ebp), %eax
-1:
-#endif
-
 	pushl	$__KERNEL_CS
 	pushl	%eax