146 lines
3.2 KiB
ArmAsm
146 lines
3.2 KiB
ArmAsm
/* linux_trampoline */
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/* SPDX-License-Identifier: GPL-2.0-only */
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/* NOTE: THIS CODE MUST REMAIN POSITION INDEPENDENT
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* IT SHOULDN'T USE THE STACK
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* AND IN GENERAL EXPECT NOTHING BUT RAM TO WORK
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*/
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.code32
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.data
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#include "linux_trampoline.h"
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#define HEADER_SIG 0x4f49424c // LBIO little endian
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#define CB_TAG_FORWARD 0x11
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#define CB_TAG_MEMORY 0x1
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#define CB_TAG_FRAMEBUFFER 0x12
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#define CB_TAG_ACPI_RSDP 0x43
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#define ACPI_RSDP_ADDR 0x70
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#define E820_NR_OFFSET 0x1e8
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#define LINUX_ENTRY_OFFSET 0x214
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#define E820_OFFSET 0x2d0
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.trampoline_start:
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cld
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xor %edx, %edx
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mov $0, %ecx
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.headerSearch:
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mov $0x10000, %ebx
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add %ecx, %ebx
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mov (%ecx), %eax
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cmp $HEADER_SIG, %eax
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je .headerSearchDone // found the header
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add $16, %ecx
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cmp %ecx, %ebx
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jne .headerSearch
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.headerSearchDone:
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cmp %ecx, %ebx // reached the end == not found anything?
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je 2f // give up
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// we assume the checksum is okay, no test
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mov 4(%ecx), %ebx
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add %ecx, %ebx // ebx = cb_header + header_bytes
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mov 20(%ecx), %ecx // ecx = table_entries
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.tableScan:
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cmp $CB_TAG_FORWARD, (%ebx)
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jne .testMemory
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/* forward tag: assume 32bit pointer */
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mov 8(%ebx), %ecx
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jmp .headerSearch
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.testMemory:
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cmp $CB_TAG_MEMORY, (%ebx)
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jne .testAcpiRsdp
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/* memory tag: copy e820 map and entry count. also determine alt_mem_k */
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mov 4(%ebx), %eax
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sub $8, %eax
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shr $2, %eax /* eax = number of dwords of e820 data */
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cmp $(32 * 5), %eax /* linux wants at most 32 entries of 5 dwords */
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jng 1f
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mov $(32 * 5), %eax /* only copy 32 entries */
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1:
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mov %eax, %esi
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mov $5, %edi
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div %edi
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mov %eax, (LINUX_PARAM_LOC + E820_NR_OFFSET)
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mov %esi, %eax
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xchg %eax, %ecx
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lea 8(%ebx), %esi /* e820 data source */
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mov $(LINUX_PARAM_LOC + E820_OFFSET), %edi
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rep movsl
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xchg %eax, %ecx
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/* e820 and LB_TAG_MEMORY type don't fully match: remap unknown type to 2, reserved memory */
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mov (LINUX_PARAM_LOC + E820_NR_OFFSET), %eax
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mov $(LINUX_PARAM_LOC + E820_OFFSET), %edi
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.test_e820_entry:
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cmp $0, %eax
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je .endScan
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cmp $12, 16(%edi) /* type */
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jng .next_e820_entry
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/* Fixup the type to 2, reserved memory */
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mov $2, 16(%edi)
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.next_e820_entry:
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dec %eax
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add $20, %edi
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jmp .test_e820_entry
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.testAcpiRsdp:
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cmp $CB_TAG_ACPI_RSDP, (%ebx)
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jne .testFramebuffer
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mov 8(%ebx), %eax
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mov %eax, (LINUX_PARAM_LOC + ACPI_RSDP_ADDR)
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mov 12(%ebx), %eax
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mov %eax, (LINUX_PARAM_LOC + ACPI_RSDP_ADDR + 4)
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jmp .endScan
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.testFramebuffer:
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cmp $CB_TAG_FRAMEBUFFER, (%ebx)
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jne .endScan
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/* TODO: handle framebuffer tag */
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.endScan:
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add 4(%ebx), %ebx
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dec %ecx
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jnz .tableScan
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/* Setup basic code and data segment selectors for Linux
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**
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** Flat code segment descriptor:
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** selector: 0x10
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** base : 0x00000000
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** limit : 0xFFFFFFFF
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** type : code, execute, read
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**
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** Flat data segment descriptor:
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** selector: 0x18
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** base : 0x00000000
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** limit : 0xFFFFFFFF
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** type : data, read/write
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**
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** Use TRAMPOLINE_ENTRY_LOC as a scratchpad.
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*/
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mov $TRAMPOLINE_ENTRY_LOC, %eax
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movl $0x0000ffff, 16(%eax) // Set up the 2 new descriptors
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movl $0x00cf9b00, 20(%eax)
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movl $0x0000ffff, 24(%eax)
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movl $0x00cf9300, 28(%eax)
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movb $0x2b, 0(%eax) // Set the size
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movl %eax, 2(%eax) // Set pointer to new GDT
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lgdt (%eax) // Load it
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/* finally: jump to kernel */
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mov $LINUX_PARAM_LOC, %esi
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jmp *(LINUX_PARAM_LOC + LINUX_ENTRY_OFFSET)
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2:
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hlt
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jmp 2b
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.trampoline_end:
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