Message-ID: <60323D2A-BB58-4112-B21C-62D7A483F2D3@zytor.com>
Date:   Wed, 15 Feb 2017 10:12:07 -0800
From:   hpa@...or.com
To:     Chao Peng <chao.p.peng@...ux.intel.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org, grub-devel@....org
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Juergen Gross <jgross@...e.com>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Borislav Petkov <bp@...e.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Thomas Garnier <thgarnie@...gle.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Amnon Ilan <ailan@...hat.com>
Subject: Re: [RFC PATCH] x86/boot: make ELF kernel multiboot-able

On February 15, 2017 6:41:56 AM PST, Chao Peng <chao.p.peng@...ux.intel.com> wrote:
>Multiboot specification
>(http://git.savannah.gnu.org/cgit/grub.git/tree/doc/multiboot.texi?h=multiboot2)
>is an open standard that provides kernels with a uniform way to be
>booted
>by multiboot-compliant bootloaders (like grub).
>
>This patch is trying to make Linux ELF kernel image to be a
>multiboot-compliant OS so that it can be loaded by a multiboot-comliant
>bootloader. The benefit is eliminating the maintainance for realmode
>and
>decompression code and especially when the kernel is loaded in a
>virtual
>machine, the reducing for these code can greatly cuts down the boot
>time.
>
>However, the current version of multiboot spec doesn't support 64 bit
>well so for 64 bit kernel we need stub code to jump from 32 bit code to
>64 bit code. Besides, there are still some other issues:
>  1). '-z max-page-size=0x1000' is used so the text segment start is in
>  multiboot header search scope because GNU LD has default page size of
>  0x00200000 for ELF64, which will fail multiboot test.
>
>2). The bootloader like grub has support for ELF kernel (even for
>ELF64)
>  which makes the patch easier. However, the current grub implementaion
>thinks the entry address should be a VA. E.g. for 64 bit kernel, the
>entry
>address (0x1000000) is actually phiscial address, grub refuses to load
>it
>  by saying: 'entry point isn't in a segment'.
>
>This patch is sent out as RFC in case you have some ideas.
>
>Signed-off-by: Chao Peng <chao.p.peng@...ux.intel.com>
>---
> arch/x86/Kconfig             |   6 +
> arch/x86/Makefile            |   4 +
> arch/x86/kernel/head64.c     |  64 ++++++-
> arch/x86/kernel/head_64.S    | 175 ++++++++++++++++++
>arch/x86/kernel/multiboot2.h | 417
>+++++++++++++++++++++++++++++++++++++++++++
> 5 files changed, 665 insertions(+), 1 deletion(-)
> create mode 100644 arch/x86/kernel/multiboot2.h
>
>diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>index bada636..75a9ef2 100644
>--- a/arch/x86/Kconfig
>+++ b/arch/x86/Kconfig
>@@ -585,6 +585,12 @@ config X86_AMD_PLATFORM_DEVICE
> 	  I2C and UART depend on COMMON_CLK to set clock. GPIO driver is
> 	  implemented under PINCTRL subsystem.
> 
>+config X86_MULTIBOOT_STUB
>+	bool "Multiboot stub support for ELF kernel image"
>+	default n
>+	---help---
>+	  Set whether multiboot stub is on or off.
>+
> config IOSF_MBI
> 	tristate "Intel SoC IOSF Sideband support for SoC platforms"
> 	depends on PCI
>diff --git a/arch/x86/Makefile b/arch/x86/Makefile
>index 2d44933..d945c34 100644
>--- a/arch/x86/Makefile
>+++ b/arch/x86/Makefile
>@@ -39,6 +39,10 @@ ifdef CONFIG_X86_NEED_RELOCS
>         LDFLAGS_vmlinux := --emit-relocs
> endif
> 
>+ifdef CONFIG_X86_MULTIBOOT_STUB
>+	LDFLAGS_vmlinux += -z max-page-size=0x1000
>+endif
>+
> #
> # Prevent GCC from generating any FP code by mistake.
> #
>diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
>index 54a2372..c0f375a 100644
>--- a/arch/x86/kernel/head64.c
>+++ b/arch/x86/kernel/head64.c
>@@ -29,6 +29,8 @@
> #include <asm/microcode.h>
> #include <asm/kasan.h>
> 
>+#include "multiboot2.h"
>+
> /*
>  * Manage page tables very early on.
>  */
>@@ -36,6 +38,7 @@ extern pgd_t early_level4_pgt[PTRS_PER_PGD];
>extern pmd_t
>early_dynamic_pgts[EARLY_DYNAMIC_PAGE_TABLES][PTRS_PER_PMD];
> static unsigned int __initdata next_early_pgt = 2;
>pmdval_t early_pmd_flags = __PAGE_KERNEL_LARGE & ~(_PAGE_GLOBAL |
>_PAGE_NX);
>+char *multiboot_info = NULL;
> 
> /* Wipe all early page tables except for the kernel symbol map */
> static void __init reset_early_page_tables(void)
>@@ -130,6 +133,60 @@ static void __init copy_bootdata(char
>*real_mode_data)
> 	}
> }
> 
>+static void __init copy_multiboot_cmdline(struct multiboot_tag_string
>*tag)
>+{
>+	unsigned int size = tag->size - 8;
>+
>+	if (size > COMMAND_LINE_SIZE)
>+		size = COMMAND_LINE_SIZE;
>+	boot_params.hdr.cmdline_size = size;
>+	memcpy(boot_command_line, tag->string, size);
>+}
>+
>+static void __init copy_multiboot_mmap(struct multiboot_tag_mmap *tag)
>+{
>+	multiboot_memory_map_t *mmap;
>+	int nr = 0;
>+
>+	for (mmap = tag->entries;
>+		(u8 *)mmap < (u8 *)tag + tag->size && nr < E820MAX;
>+		mmap = (multiboot_memory_map_t *)((unsigned long)mmap +
>+							tag->entry_size)) {
>+		boot_params.e820_map[nr].addr = mmap->addr;
>+		boot_params.e820_map[nr].size = mmap->len;
>+		boot_params.e820_map[nr].type = mmap->type;
>+		nr++;
>+	}
>+	boot_params.e820_entries = nr;
>+}
>+
>+static void __init copy_multiboot_info(void)
>+{
>+	struct multiboot_tag *tag;
>+	char *ptr = __va(multiboot_info);
>+
>+	boot_params.hdr.boot_flag = 0xAA55;
>+	boot_params.hdr.header = 0x53726448;
>+	boot_params.hdr.version = 0x202;
>+
>+	for (tag = (struct multiboot_tag *)(ptr + 8);
>+		tag->type != MULTIBOOT_TAG_TYPE_END;
>+		tag = (struct multiboot_tag *)((u8 *) tag +
>+						((tag->size + 7) & ~7))) {
>+		switch (tag->type) {
>+		case MULTIBOOT_TAG_TYPE_CMDLINE:
>+			copy_multiboot_cmdline(
>+					(struct multiboot_tag_string *)tag);
>+			break;
>+		case MULTIBOOT_TAG_TYPE_MMAP:
>+			copy_multiboot_mmap((struct multiboot_tag_mmap *)tag);
>+			break;
>+		default:
>+			break;
>+		}
>+	}
>+}
>+
>asmlinkage __visible void __init x86_64_start_kernel(char *
>real_mode_data)
> {
> 	int i;
>@@ -163,7 +220,12 @@ asmlinkage __visible void __init
>x86_64_start_kernel(char * real_mode_data)
> 		set_intr_gate(i, early_idt_handler_array[i]);
> 	load_idt((const struct desc_ptr *)&idt_descr);
> 
>-	copy_bootdata(__va(real_mode_data));
>+	if (multiboot_info) {
>+		copy_multiboot_info();
>+		real_mode_data = (char *) (__pa(&boot_params));
>+	} else {
>+		copy_bootdata(__va(real_mode_data));
>+	}
> 
> 	/*
> 	 * Load microcode early on BSP.
>diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
>index b4421cc..1c944fd 100644
>--- a/arch/x86/kernel/head_64.S
>+++ b/arch/x86/kernel/head_64.S
>@@ -22,6 +22,7 @@
> #include <asm/nops.h>
> #include "../entry/calling.h"
> #include <asm/export.h>
>+#include <asm/boot.h>
> 
> #ifdef CONFIG_PARAVIRT
> #include <asm/asm-offsets.h>
>@@ -32,6 +33,8 @@
> #define INTERRUPT_RETURN iretq
> #endif
> 
>+#include "multiboot2.h"
>+
>/* we are not able to switch in one step to the final KERNEL ADDRESS
>SPACE
>  * because we need identity-mapped pages.
>  *
>@@ -66,6 +69,161 @@ startup_64:
> 	 * tables and then reload them.
> 	 */
> 
>+#ifdef CONFIG_X86_MULTIBOOT_STUB
>+	jmp multiboot_entry
>+
>+	.align MULTIBOOT_HEADER_ALIGN
>+multiboot_header:
>+        /* magic */
>+        .long	MULTIBOOT2_HEADER_MAGIC
>+        /* ISA: i386 */
>+        .long	MULTIBOOT_ARCHITECTURE_I386
>+        /* Header length.  */
>+        .long	multiboot_header_end - multiboot_header
>+        /* checksum */
>+        .long	-(MULTIBOOT2_HEADER_MAGIC + MULTIBOOT_ARCHITECTURE_I386
>+ \
>+		(multiboot_header_end - multiboot_header))
>+
>+	/* information request tag */
>+infomation_request_start:
>+	.short	MULTIBOOT_HEADER_TAG_INFORMATION_REQUEST
>+	.short	MULTIBOOT_HEADER_TAG_OPTIONAL
>+	.long	infomation_request_end - infomation_request_start
>+	.long	MULTIBOOT_TAG_TYPE_CMDLINE
>+	.long	MULTIBOOT_TAG_TYPE_MMAP
>+infomation_request_end:
>+
>+	.short MULTIBOOT_HEADER_TAG_END
>+	.short 0
>+	.long 8
>+multiboot_header_end:
>+
>+multiboot_entry:
>+	cmpl	$MULTIBOOT2_BOOTLOADER_MAGIC, %eax
>+	jne	not_multiboot
>+
>+	.code32
>+	/* %ebp contains the address we are loaded at by the boot loader */
>+	movl $_text - __START_KERNEL_map, %ebp
>+
>+	/* Store the Multiboot information structure */
>+	movl	%ebx, (multiboot_info - startup_64)(%ebp)
>+
>+	/*
>+	 * The Multiboot-compliant bootloader puts the machine in 32-bit mode
>+	 * so we need to switch to long mode by ourself. If later version of
>+	 * Multiboot protocal supports 64-bit machine state then this part of
>+	 * code may be eliminated.
>+	 */
>+	cli
>+	movl	$(__BOOT_DS), %eax
>+	movl	%eax, %ds
>+	movl	%eax, %es
>+	movl	%eax, %ss
>+
>+	/* Setup a stack and make sure cpu supports long mode. */
>+	movl	(__end_init_task - 8 - startup_64)(%ebp), %eax
>+	movl	%eax, %esp
>+
>+	call	verify_cpu
>+	testl	%eax, %eax
>+	jnz	no_longmode
>+
>+/*
>+ * Prepare for entering 64 bit mode
>+ */
>+
>+	/* Load new GDT with the 64bit segments using 32bit descriptor */
>+	leal	(gdt - startup_64)(%ebp), %eax
>+	movl	%eax, (gdt + 2 - startup_64)(%ebp)
>+	lgdt	(gdt - startup_64)(%ebp)
>+
>+	/* Enable PAE mode */
>+	movl	%cr4, %eax
>+	orl	$X86_CR4_PAE, %eax
>+	movl	%eax, %cr4
>+
>+ /*
>+  * Build early 4G boot pagetable
>+  */
>+	/* Initialize Page tables to 0 */
>+	/*leal	pgtable(%ebp), %edi
>+	xorl	%eax, %eax
>+	movl	$(BOOT_INIT_PGT_SIZE/4), %ecx
>+	rep	stosl*/
>+
>+	/* Build Level 4 */
>+	leal	(pgtable - startup_64)(%ebp), %edi
>+	leal	0x1007 (%edi), %eax
>+	movl	%eax, 0(%edi)
>+
>+	/* Build Level 3 */
>+	leal	(pgtable + 0x1000 - startup_64)(%ebp), %edi
>+	leal	0x1007(%edi), %eax
>+	movl	$4, %ecx
>+1:	movl	%eax, 0x00(%edi)
>+	addl	$0x00001000, %eax
>+	addl	$8, %edi
>+	decl	%ecx
>+	jnz	1b
>+
>+	/* Build Level 2 */
>+	leal	(pgtable + 0x2000 - startup_64)(%ebp), %edi
>+	movl	$0x00000183, %eax
>+	movl	$2048, %ecx
>+1:	movl	%eax, 0(%edi)
>+	addl	$0x00200000, %eax
>+	addl	$8, %edi
>+	decl	%ecx
>+	jnz	1b
>+
>+	/* Enable the boot page tables */
>+	leal	(pgtable - startup_64)(%ebp), %eax
>+	movl	%eax, %cr3
>+
>+	/* Enable Long mode in EFER (Extended Feature Enable Register) */
>+	movl	$MSR_EFER, %ecx
>+	rdmsr
>+	btsl	$_EFER_LME, %eax
>+	wrmsr
>+
>+	/* After gdt is loaded */
>+	xorl	%eax, %eax
>+	lldt	%ax
>+	movl    $__BOOT_TSS, %eax
>+	ltr	%ax
>+
>+	/*
>+	 * Setup for the jump to 64bit mode
>+	 *
>+	 * When the jump is performend we will be in long mode but
>+	 * in 32bit compatibility mode with EFER.LME = 1, CS.L = 0, CS.D = 1
>+	 * (and in turn EFER.LMA = 1).	To jump into 64bit mode we use
>+	 * the new gdt/idt that has __KERNEL_CS with CS.L = 1.
>+	 * We place all of the values on our mini stack so lret can
>+	 * used to perform that far jump.
>+	 */
>+	pushl	$__KERNEL_CS
>+	leal	(not_multiboot - startup_64)(%ebp), %eax
>+	pushl	%eax
>+
>+	/* Enter paged protected Mode, activating Long Mode */
>+	movl	$(X86_CR0_PG | X86_CR0_PE), %eax /* Enable Paging and Protected
>mode */
>+	movl	%eax, %cr0
>+
>+	/* Jump from 32bit compatibility mode into 64bit mode. */
>+	lret
>+
>+no_longmode:
>+	/* This isn't an x86-64 CPU so hang */
>+1:
>+	hlt
>+	jmp     1b
>+
>+	.code64
>+not_multiboot:
>+#endif
>+
> 	/*
>	 * Setup stack for verify_cpu(). "-8" because initial_stack is defined
> 	 * this way, see below. Our best guess is a NULL ptr for stack
>@@ -489,6 +647,23 @@ ENTRY(phys_base)
> 	.quad   0x0000000000000000
> EXPORT_SYMBOL(phys_base)
> 
>+#ifdef CONFIG_X86_MULTIBOOT_STUB
>+gdt:
>+	.word	gdt_end - gdt
>+	.long	0
>+	.word	0
>+	.quad	0x0000000000000000	/* NULL descriptor */
>+	.quad	0x00af9a000000ffff	/* __KERNEL_CS */
>+	.quad	0x00cf92000000ffff	/* __KERNEL_DS */
>+	.quad	0x0080890000000000	/* TS descriptor */
>+	.quad   0x0000000000000000	/* TS continued */
>+gdt_end:
>+
>+	.balign 4096
>+pgtable:
>+	.fill BOOT_PGT_SIZE, 1, 0
>+#endif
>+
> #include "../../x86/xen/xen-head.S"
> 	
> 	__PAGE_ALIGNED_BSS
>diff --git a/arch/x86/kernel/multiboot2.h
>b/arch/x86/kernel/multiboot2.h
>new file mode 100644
>index 0000000..03bb5d2
>--- /dev/null
>+++ b/arch/x86/kernel/multiboot2.h
>@@ -0,0 +1,417 @@
>+/*  multiboot2.h - Multiboot 2 header file.  */
>+/*  Copyright (C) 1999,2003,2007,2008,2009,2010  Free Software
>Foundation, Inc.
>+ *
>+ *  Permission is hereby granted, free of charge, to any person
>obtaining a copy
>+ *  of this software and associated documentation files (the
>"Software"), to
>+ *  deal in the Software without restriction, including without
>limitation the
>+ *  rights to use, copy, modify, merge, publish, distribute,
>sublicense, and/or
>+ *  sell copies of the Software, and to permit persons to whom the
>Software is
>+ *  furnished to do so, subject to the following conditions:
>+ *
>+ *  The above copyright notice and this permission notice shall be
>included in
>+ *  all copies or substantial portions of the Software.
>+ *
>+ *  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
>EXPRESS OR
>+ *  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
>MERCHANTABILITY,
>+ *  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT
>SHALL ANY
>+ *  DEVELOPER OR DISTRIBUTOR BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
>LIABILITY,
>+ *  WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
>OUT OF OR
>+ *  IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
>THE SOFTWARE.
>+ */
>+
>+#ifndef _ARCH_X86_KERNEL_MULTIBOOT2
>+#define _ARCH_X86_KERNEL_MULTIBOOT2
>+
>+/* How many bytes from the start of the file we search for the header.
> */
>+#define MULTIBOOT_SEARCH			32768
>+#define MULTIBOOT_HEADER_ALIGN			8
>+
>+/* The magic field should contain this.  */
>+#define MULTIBOOT2_HEADER_MAGIC			0xe85250d6
>+
>+/* This should be in %eax.  */
>+#define MULTIBOOT2_BOOTLOADER_MAGIC		0x36d76289
>+
>+/* Alignment of multiboot modules.  */
>+#define MULTIBOOT_MOD_ALIGN			0x00001000
>+
>+/* Alignment of the multiboot info structure.  */
>+#define MULTIBOOT_INFO_ALIGN			0x00000008
>+
>+/* Flags set in the 'flags' member of the multiboot header.  */
>+
>+#define MULTIBOOT_TAG_ALIGN                  8
>+#define MULTIBOOT_TAG_TYPE_END               0
>+#define MULTIBOOT_TAG_TYPE_CMDLINE           1
>+#define MULTIBOOT_TAG_TYPE_BOOT_LOADER_NAME  2
>+#define MULTIBOOT_TAG_TYPE_MODULE            3
>+#define MULTIBOOT_TAG_TYPE_BASIC_MEMINFO     4
>+#define MULTIBOOT_TAG_TYPE_BOOTDEV           5
>+#define MULTIBOOT_TAG_TYPE_MMAP              6
>+#define MULTIBOOT_TAG_TYPE_VBE               7
>+#define MULTIBOOT_TAG_TYPE_FRAMEBUFFER       8
>+#define MULTIBOOT_TAG_TYPE_ELF_SECTIONS      9
>+#define MULTIBOOT_TAG_TYPE_APM               10
>+#define MULTIBOOT_TAG_TYPE_EFI32             11
>+#define MULTIBOOT_TAG_TYPE_EFI64             12
>+#define MULTIBOOT_TAG_TYPE_SMBIOS            13
>+#define MULTIBOOT_TAG_TYPE_ACPI_OLD          14
>+#define MULTIBOOT_TAG_TYPE_ACPI_NEW          15
>+#define MULTIBOOT_TAG_TYPE_NETWORK           16
>+#define MULTIBOOT_TAG_TYPE_EFI_MMAP          17
>+#define MULTIBOOT_TAG_TYPE_EFI_BS            18
>+#define MULTIBOOT_TAG_TYPE_EFI32_IH          19
>+#define MULTIBOOT_TAG_TYPE_EFI64_IH          20
>+#define MULTIBOOT_TAG_TYPE_LOAD_BASE_ADDR    21
>+
>+#define MULTIBOOT_HEADER_TAG_END  0
>+#define MULTIBOOT_HEADER_TAG_INFORMATION_REQUEST  1
>+#define MULTIBOOT_HEADER_TAG_ADDRESS  2
>+#define MULTIBOOT_HEADER_TAG_ENTRY_ADDRESS  3
>+#define MULTIBOOT_HEADER_TAG_CONSOLE_FLAGS  4
>+#define MULTIBOOT_HEADER_TAG_FRAMEBUFFER  5
>+#define MULTIBOOT_HEADER_TAG_MODULE_ALIGN  6
>+#define MULTIBOOT_HEADER_TAG_EFI_BS        7
>+#define MULTIBOOT_HEADER_TAG_ENTRY_ADDRESS_EFI32  8
>+#define MULTIBOOT_HEADER_TAG_ENTRY_ADDRESS_EFI64  9
>+#define MULTIBOOT_HEADER_TAG_RELOCATABLE  10
>+
>+#define MULTIBOOT_ARCHITECTURE_I386  0
>+#define MULTIBOOT_ARCHITECTURE_MIPS32  4
>+#define MULTIBOOT_HEADER_TAG_OPTIONAL 1
>+
>+#define MULTIBOOT_LOAD_PREFERENCE_NONE 0
>+#define MULTIBOOT_LOAD_PREFERENCE_LOW 1
>+#define MULTIBOOT_LOAD_PREFERENCE_HIGH 2
>+
>+#define MULTIBOOT_CONSOLE_FLAGS_CONSOLE_REQUIRED 1
>+#define MULTIBOOT_CONSOLE_FLAGS_EGA_TEXT_SUPPORTED 2
>+
>+#ifndef __ASSEMBLY__
>+
>+typedef unsigned char		multiboot_uint8_t;
>+typedef unsigned short		multiboot_uint16_t;
>+typedef unsigned int		multiboot_uint32_t;
>+typedef unsigned long long	multiboot_uint64_t;
>+
>+struct multiboot_header
>+{
>+  /* Must be MULTIBOOT_MAGIC - see above.  */
>+  multiboot_uint32_t magic;
>+
>+  /* ISA */
>+  multiboot_uint32_t architecture;
>+
>+  /* Total header length.  */
>+  multiboot_uint32_t header_length;
>+
>+  /* The above fields plus this one must equal 0 mod 2^32. */
>+  multiboot_uint32_t checksum;
>+};
>+
>+struct multiboot_header_tag
>+{
>+  multiboot_uint16_t type;
>+  multiboot_uint16_t flags;
>+  multiboot_uint32_t size;
>+};
>+
>+struct multiboot_header_tag_information_request
>+{
>+  multiboot_uint16_t type;
>+  multiboot_uint16_t flags;
>+  multiboot_uint32_t size;
>+  multiboot_uint32_t requests[0];
>+};
>+
>+struct multiboot_header_tag_address
>+{
>+  multiboot_uint16_t type;
>+  multiboot_uint16_t flags;
>+  multiboot_uint32_t size;
>+  multiboot_uint32_t header_addr;
>+  multiboot_uint32_t load_addr;
>+  multiboot_uint32_t load_end_addr;
>+  multiboot_uint32_t bss_end_addr;
>+};
>+
>+struct multiboot_header_tag_entry_address
>+{
>+  multiboot_uint16_t type;
>+  multiboot_uint16_t flags;
>+  multiboot_uint32_t size;
>+  multiboot_uint32_t entry_addr;
>+};
>+
>+struct multiboot_header_tag_console_flags
>+{
>+  multiboot_uint16_t type;
>+  multiboot_uint16_t flags;
>+  multiboot_uint32_t size;
>+  multiboot_uint32_t console_flags;
>+};
>+
>+struct multiboot_header_tag_framebuffer
>+{
>+  multiboot_uint16_t type;
>+  multiboot_uint16_t flags;
>+  multiboot_uint32_t size;
>+  multiboot_uint32_t width;
>+  multiboot_uint32_t height;
>+  multiboot_uint32_t depth;
>+};
>+
>+struct multiboot_header_tag_module_align
>+{
>+  multiboot_uint16_t type;
>+  multiboot_uint16_t flags;
>+  multiboot_uint32_t size;
>+};
>+
>+struct multiboot_header_tag_relocatable
>+{
>+  multiboot_uint16_t type;
>+  multiboot_uint16_t flags;
>+  multiboot_uint32_t size;
>+  multiboot_uint32_t min_addr;
>+  multiboot_uint32_t max_addr;
>+  multiboot_uint32_t align;
>+  multiboot_uint32_t preference;
>+};
>+
>+struct multiboot_color
>+{
>+  multiboot_uint8_t red;
>+  multiboot_uint8_t green;
>+  multiboot_uint8_t blue;
>+};
>+
>+struct multiboot_mmap_entry
>+{
>+  multiboot_uint64_t addr;
>+  multiboot_uint64_t len;
>+#define MULTIBOOT_MEMORY_AVAILABLE		1
>+#define MULTIBOOT_MEMORY_RESERVED		2
>+#define MULTIBOOT_MEMORY_ACPI_RECLAIMABLE       3
>+#define MULTIBOOT_MEMORY_NVS                    4
>+#define MULTIBOOT_MEMORY_BADRAM                 5
>+  multiboot_uint32_t type;
>+  multiboot_uint32_t zero;
>+};
>+typedef struct multiboot_mmap_entry multiboot_memory_map_t;
>+
>+struct multiboot_tag
>+{
>+  multiboot_uint32_t type;
>+  multiboot_uint32_t size;
>+};
>+
>+struct multiboot_tag_string
>+{
>+  multiboot_uint32_t type;
>+  multiboot_uint32_t size;
>+  char string[0];
>+};
>+
>+struct multiboot_tag_module
>+{
>+  multiboot_uint32_t type;
>+  multiboot_uint32_t size;
>+  multiboot_uint32_t mod_start;
>+  multiboot_uint32_t mod_end;
>+  char cmdline[0];
>+};
>+
>+struct multiboot_tag_basic_meminfo
>+{
>+  multiboot_uint32_t type;
>+  multiboot_uint32_t size;
>+  multiboot_uint32_t mem_lower;
>+  multiboot_uint32_t mem_upper;
>+};
>+
>+struct multiboot_tag_bootdev
>+{
>+  multiboot_uint32_t type;
>+  multiboot_uint32_t size;
>+  multiboot_uint32_t biosdev;
>+  multiboot_uint32_t slice;
>+  multiboot_uint32_t part;
>+};
>+
>+struct multiboot_tag_mmap
>+{
>+  multiboot_uint32_t type;
>+  multiboot_uint32_t size;
>+  multiboot_uint32_t entry_size;
>+  multiboot_uint32_t entry_version;
>+  struct multiboot_mmap_entry entries[0];
>+};
>+
>+struct multiboot_vbe_info_block
>+{
>+  multiboot_uint8_t external_specification[512];
>+};
>+
>+struct multiboot_vbe_mode_info_block
>+{
>+  multiboot_uint8_t external_specification[256];
>+};
>+
>+struct multiboot_tag_vbe
>+{
>+  multiboot_uint32_t type;
>+  multiboot_uint32_t size;
>+
>+  multiboot_uint16_t vbe_mode;
>+  multiboot_uint16_t vbe_interface_seg;
>+  multiboot_uint16_t vbe_interface_off;
>+  multiboot_uint16_t vbe_interface_len;
>+
>+  struct multiboot_vbe_info_block vbe_control_info;
>+  struct multiboot_vbe_mode_info_block vbe_mode_info;
>+};
>+
>+struct multiboot_tag_framebuffer_common
>+{
>+  multiboot_uint32_t type;
>+  multiboot_uint32_t size;
>+
>+  multiboot_uint64_t framebuffer_addr;
>+  multiboot_uint32_t framebuffer_pitch;
>+  multiboot_uint32_t framebuffer_width;
>+  multiboot_uint32_t framebuffer_height;
>+  multiboot_uint8_t framebuffer_bpp;
>+#define MULTIBOOT_FRAMEBUFFER_TYPE_INDEXED 0
>+#define MULTIBOOT_FRAMEBUFFER_TYPE_RGB     1
>+#define MULTIBOOT_FRAMEBUFFER_TYPE_EGA_TEXT	2
>+  multiboot_uint8_t framebuffer_type;
>+  multiboot_uint16_t reserved;
>+};
>+
>+struct multiboot_tag_framebuffer
>+{
>+  struct multiboot_tag_framebuffer_common common;
>+
>+  union
>+  {
>+    struct
>+    {
>+      multiboot_uint16_t framebuffer_palette_num_colors;
>+      struct multiboot_color framebuffer_palette[0];
>+    };
>+    struct
>+    {
>+      multiboot_uint8_t framebuffer_red_field_position;
>+      multiboot_uint8_t framebuffer_red_mask_size;
>+      multiboot_uint8_t framebuffer_green_field_position;
>+      multiboot_uint8_t framebuffer_green_mask_size;
>+      multiboot_uint8_t framebuffer_blue_field_position;
>+      multiboot_uint8_t framebuffer_blue_mask_size;
>+    };
>+  };
>+};
>+
>+struct multiboot_tag_elf_sections
>+{
>+  multiboot_uint32_t type;
>+  multiboot_uint32_t size;
>+  multiboot_uint32_t num;
>+  multiboot_uint32_t entsize;
>+  multiboot_uint32_t shndx;
>+  char sections[0];
>+};
>+
>+struct multiboot_tag_apm
>+{
>+  multiboot_uint32_t type;
>+  multiboot_uint32_t size;
>+  multiboot_uint16_t version;
>+  multiboot_uint16_t cseg;
>+  multiboot_uint32_t offset;
>+  multiboot_uint16_t cseg_16;
>+  multiboot_uint16_t dseg;
>+  multiboot_uint16_t flags;
>+  multiboot_uint16_t cseg_len;
>+  multiboot_uint16_t cseg_16_len;
>+  multiboot_uint16_t dseg_len;
>+};
>+
>+struct multiboot_tag_efi32
>+{
>+  multiboot_uint32_t type;
>+  multiboot_uint32_t size;
>+  multiboot_uint32_t pointer;
>+};
>+
>+struct multiboot_tag_efi64
>+{
>+  multiboot_uint32_t type;
>+  multiboot_uint32_t size;
>+  multiboot_uint64_t pointer;
>+};
>+
>+struct multiboot_tag_smbios
>+{
>+  multiboot_uint32_t type;
>+  multiboot_uint32_t size;
>+  multiboot_uint8_t major;
>+  multiboot_uint8_t minor;
>+  multiboot_uint8_t reserved[6];
>+  multiboot_uint8_t tables[0];
>+};
>+
>+struct multiboot_tag_old_acpi
>+{
>+  multiboot_uint32_t type;
>+  multiboot_uint32_t size;
>+  multiboot_uint8_t rsdp[0];
>+};
>+
>+struct multiboot_tag_new_acpi
>+{
>+  multiboot_uint32_t type;
>+  multiboot_uint32_t size;
>+  multiboot_uint8_t rsdp[0];
>+};
>+
>+struct multiboot_tag_network
>+{
>+  multiboot_uint32_t type;
>+  multiboot_uint32_t size;
>+  multiboot_uint8_t dhcpack[0];
>+};
>+
>+struct multiboot_tag_efi_mmap
>+{
>+  multiboot_uint32_t type;
>+  multiboot_uint32_t size;
>+  multiboot_uint32_t descr_size;
>+  multiboot_uint32_t descr_vers;
>+  multiboot_uint8_t efi_mmap[0];
>+};
>+
>+struct multiboot_tag_efi32_ih
>+{
>+  multiboot_uint32_t type;
>+  multiboot_uint32_t size;
>+  multiboot_uint32_t pointer;
>+};
>+
>+struct multiboot_tag_efi64_ih
>+{
>+  multiboot_uint32_t type;
>+  multiboot_uint32_t size;
>+  multiboot_uint64_t pointer;
>+};
>+
>+struct multiboot_tag_load_base_addr
>+{
>+  multiboot_uint32_t type;
>+  multiboot_uint32_t size;
>+  multiboot_uint32_t load_base_addr;
>+};
>+
>+#endif /* __ASSEMBLY__ */
>+
>+#endif /* _ARCH_X86_KERNEL_MULTIBOOT2 */

As has been shown many times before, this is a really bad idea.  Unless there is a real-life use case where this matters enormously, this is nacked with extreme prejudice.
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.

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