1/* linux_trampoline */ 2/* SPDX-License-Identifier: GPL-2.0-only */ 3 4/* NOTE: THIS CODE MUST REMAIN POSITION INDEPENDENT 5 * IT SHOULDN'T USE THE STACK 6 * AND IN GENERAL EXPECT NOTHING BUT RAM TO WORK 7 */ 8.code32 9.data 10 11#include "linux_trampoline.h" 12#define HEADER_SIG 0x4f49424c // LBIO little endian 13#define CB_TAG_FORWARD 0x11 14#define CB_TAG_MEMORY 0x1 15#define CB_TAG_FRAMEBUFFER 0x12 16#define CB_TAG_ACPI_RSDP 0x43 17 18#define ACPI_RSDP_ADDR 0x70 19#define E820_NR_OFFSET 0x1e8 20#define PROTOCOL_VERSION 0x206 21#define LINUX_ENTRY_OFFSET 0x214 22#define E820_OFFSET 0x2d0 23 24.trampoline_start: 25cld 26xor %edx, %edx 27mov $0, %ecx 28 29.headerSearch: 30mov $0x10000, %ebx 31add %ecx, %ebx 32mov (%ecx), %eax 33cmp $HEADER_SIG, %eax 34je .headerSearchDone // found the header 35add $16, %ecx 36cmp %ecx, %ebx 37jne .headerSearch 38 39.headerSearchDone: 40cmp %ecx, %ebx // reached the end == not found anything? 41je 2f // give up 42 43// we assume the checksum is okay, no test 44mov 4(%ecx), %ebx 45add %ecx, %ebx // ebx = cb_header + header_bytes 46mov 20(%ecx), %ecx // ecx = table_entries 47 48.tableScan: 49cmp $CB_TAG_FORWARD, (%ebx) 50jne .testMemory 51 52/* forward tag: assume 32bit pointer */ 53mov 8(%ebx), %ecx 54jmp .headerSearch 55 56.testMemory: 57cmp $CB_TAG_MEMORY, (%ebx) 58jne .testAcpiRsdp 59 60/* memory tag: copy e820 map and entry count. also determine alt_mem_k */ 61mov 4(%ebx), %eax 62sub $8, %eax 63shr $2, %eax /* eax = number of dwords of e820 data */ 64/* 65 * Historically linux had space for 32 entries. This limit was increased in 66 * the year 2005 (Linux 2.6.11) to hold up to 128 entries. 67 * Assume 128 entries when the boot protocol version is 2.04+. 68 */ 69cmpw $0x0204, (LINUX_PARAM_LOC + PROTOCOL_VERSION) 70jge .e820big /* protocol version >= 2.04 can handle 128 entries of 5 dwords */ 71cmp $(32 * 5), %eax /* linux wants at most 32 entries of 5 dwords */ 72jng 1f 73mov $(32 * 5), %eax /* only copy 32 entries */ 74jmp 1f 75 76.e820big: 77cmp $(128 * 5), %eax /* linux wants at most 128 entries of 5 dwords */ 78jng 1f 79mov $(128 * 5), %eax /* only copy 128 entries */ 801: 81mov %eax, %esi 82mov $5, %edi 83div %edi 84mov %eax, (LINUX_PARAM_LOC + E820_NR_OFFSET) 85mov %esi, %eax 86xchg %eax, %ecx 87lea 8(%ebx), %esi /* e820 data source */ 88mov $(LINUX_PARAM_LOC + E820_OFFSET), %edi 89rep movsl 90xchg %eax, %ecx 91/* e820 and LB_TAG_MEMORY type don't fully match: remap unknown type to 2, reserved memory */ 92mov (LINUX_PARAM_LOC + E820_NR_OFFSET), %eax 93mov $(LINUX_PARAM_LOC + E820_OFFSET), %edi 94.test_e820_entry: 95cmp $0, %eax 96je .endScan 97cmp $12, 16(%edi) /* type */ 98jng .next_e820_entry 99/* Fixup the type to 2, reserved memory */ 100movl $2, 16(%edi) 101.next_e820_entry: 102dec %eax 103add $20, %edi 104jmp .test_e820_entry 105 106.testAcpiRsdp: 107cmp $CB_TAG_ACPI_RSDP, (%ebx) 108jne .testFramebuffer 109 110mov 8(%ebx), %eax 111mov %eax, (LINUX_PARAM_LOC + ACPI_RSDP_ADDR) 112mov 12(%ebx), %eax 113mov %eax, (LINUX_PARAM_LOC + ACPI_RSDP_ADDR + 4) 114jmp .endScan 115 116.testFramebuffer: 117cmp $CB_TAG_FRAMEBUFFER, (%ebx) 118jne .endScan 119 120cmpw $0x020f, (LINUX_PARAM_LOC + PROTOCOL_VERSION) 121jge .framebufferSetup /* protocol version >= 2.15 can handle 64-bit address */ 122cmpl $0, 0x0c(%ebx) /* check if upper 32-bit of framebuffer address are 0 */ 123jne .endScan 124 125.framebufferSetup: 126mov $LINUX_PARAM_LOC, %edi /* translate the framebuffer entry into Linux' struct screen_info */ 127mov 0x08(%ebx), %eax /* physical_address */ 128mov %eax, 0x18(%edi) /* -> lfb_base */ 129mov 0x0c(%ebx), %eax /* physical_address */ 130mov %eax, 0x3a(%edi) /* -> ext_lfb_base */ 131mov 0x10(%ebx), %eax /* x_resolution */ 132mov %ax, 0x12(%edi) /* -> lfb_width */ 133mov 0x14(%ebx), %eax /* y_resolution */ 134mov %ax, 0x14(%edi) /* -> lfb_height */ 135mov 0x18(%ebx), %edx /* bytes_per_line */ 136mov %dx, 0x24(%edi) /* -> lfb_linelength */ 137 138mul %edx /* bytes_per_line * y_resolution */ 139mov %eax, 0x1c(%edi) /* -> lfb_size */ 140 141movzbw 0x1c(%ebx), %ax /* bits_per_pixel */ 142mov %ax, 0x16(%edi) /* -> lfb_depth */ 143 144mov $4, %esi /* Copy 4 color components' pos and size, each 1 byte. */ 1451: 146mov 0x1b(%ebx, %esi, 2), %ax 147rol %ax /* Order is reversed for Linux, hence swap. */ 148mov %ax, 0x24(%edi, %esi, 2) 149dec %esi 150jnz 1b 151 152#define VIDEO_CAPABILITY_64BIT_BASE (1 << 1) 153movl $VIDEO_CAPABILITY_64BIT_BASE, 0x36(%edi) 154 155#define LFB_EFI_SIMPLE 0x70 /* VIDEO_TYPE_EFI in Linux */ 156movb $LFB_EFI_SIMPLE, 0x0f(%edi) /* -> orig_video_isVGA */ 157 158.endScan: 159add 4(%ebx), %ebx 160dec %ecx 161jnz .tableScan 162 163/* Setup basic code and data segment selectors for Linux 164** 165** Flat code segment descriptor: 166** selector: 0x10 167** base : 0x00000000 168** limit : 0xFFFFFFFF 169** type : code, execute, read 170** 171** Flat data segment descriptor: 172** selector: 0x18 173** base : 0x00000000 174** limit : 0xFFFFFFFF 175** type : data, read/write 176** 177** Use TRAMPOLINE_ENTRY_LOC as a scratchpad. 178*/ 179mov $TRAMPOLINE_ENTRY_LOC, %eax 180movl $0x0000ffff, 16(%eax) // Set up the 2 new descriptors 181movl $0x00cf9b00, 20(%eax) 182movl $0x0000ffff, 24(%eax) 183movl $0x00cf9300, 28(%eax) 184movb $0x2b, 0(%eax) // Set the size 185movl %eax, 2(%eax) // Set pointer to new GDT 186lgdt (%eax) // Load it 187 188/* finally: jump to kernel */ 189mov $LINUX_PARAM_LOC, %esi 190jmp *(LINUX_PARAM_LOC + LINUX_ENTRY_OFFSET) 191 192 1932: 194hlt 195jmp 2b 196.trampoline_end: 197