Calculate kernel size in pages dinamically
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@@ -1,3 +1,7 @@
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# This Source Code Form is subject to the terms of the Mozilla Public License,
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# v. 2.0. If a copy of the MPL was not distributed with this file, You can
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# obtain one at https://mozilla.org/MPL/2.0/.
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.section .rodata
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panic_message: .asciz "\nPanic"
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@@ -6,29 +10,41 @@ panic_message: .asciz "\nPanic"
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.section .text.boot
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.type _start, @function
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.global _start
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.globl _start
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_start:
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# Save the physical start of the text section.
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# This must be the first instruction.
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auipc s2, 0
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mv s1, a1 # Save the device tree pointer.
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auipc s2, 0 # Save the physical start of the text section.
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# Virtual address of the start of the text section.
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lui s3, %hi(_start)
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addi s3, s3, %lo(_start)
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sub s4, s3, s2 # Save the offset to the virtual memory address.
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# Set the stack pointer.
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lui sp, %hi(end)
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addi sp, sp, %lo(end)
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li t0, 64 * 1024
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add sp, sp, t0
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mv a0, s4
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call set_up_stack
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# Create identity mapping for the kernel and copy it to higher half.
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mv a0, s2
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mv a1, sp
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call get_page_tables_size
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mv a3, a0
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mv a0, s2
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mv a1, sp
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mv a2, s4
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jal imem_initialize
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mv a0, s3
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jal imem_mirror
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la a0, __bss_start
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la a1, end
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call bzero
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call set_up_trap_routine
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add sp, sp, s4 # Move the stack to the virtual memory.
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lui t0, %hi(trap_routine)
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addi t0, t0, %lo(trap_routine)
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csrw stvec, t0
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# Zero BSS and kernel stack.
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lui a0, %hi(__bss_start)
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addi a0, a0, %lo(__bss_start)
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mv a1, sp
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call imem_zero
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mv a0, sp
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mv a1, s1
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@@ -36,7 +52,74 @@ _start:
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lui t0, %hi(kernel_main)
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jr t0, %lo(kernel_main)
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# Given the start and end addresses of the kernel, calculates how
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# much additional storage should be mapped to store the page tables.
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# The original formula for getting the number of required pages is:
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# ((sp - start) / 4096 + 1 + 1023) / 1024.
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#
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# - sp is the stack pointer (kernel end).
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# - The kernel size is divided by the page size. It is the kernel size in pages.
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# - One additional page is added for the root page table.
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# - 1023 is added so the next division is rounded up and not down.
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#
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# A simplified formula is used in the actual implementation.
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#
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# Parameters:
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# a0 - Kernel start address.
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# a1 - Kernel end address.
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#
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# Returns the result in a0.
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.type get_page_tables_size, @function
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get_page_tables_size:
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sub a0, a1, a0
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srli a0, a0, 22
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addi a0, a0, 1
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slli a0, a0, 12
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ret
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# Set an exception handler.
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.type set_up_trap_routine, @function
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set_up_trap_routine:
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lui t0, %hi(trap_routine)
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addi t0, t0, %lo(trap_routine)
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csrw stvec, t0
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ret
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.type set_up_stack, @function
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set_up_stack:
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lui sp, %hi(end)
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addi sp, sp, %lo(end)
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sub sp, sp, a0
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li t0, 64 * 1024
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add sp, sp, t0
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ret
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# Zeroes a chunk of memory.
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#
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# Parameters:
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# a0 - Memory start pointer.
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# a1 - Memory end pointer.
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.type imem_zero, @function
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imem_zero:
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mv t0, a0
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.Limem_zero_loop:
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bgeu t0, a1, .Limem_zero_end
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sw zero, (t0)
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addi t0, t0, 4
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j .Limem_zero_loop
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.Limem_zero_end:
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ret
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# Copies the identity mappings to the higher half.
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#
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# Parameters:
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# a0 - Start of the kernel virtual space.
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.type imem_mirror, @function
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imem_mirror:
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csrr t0, satp
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@@ -57,8 +140,7 @@ imem_mirror:
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blt t1, t2, .Limem_mirror_copy
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# Persist the copied page table in the root page table.
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lui t4, %hi(_start)
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srli t4, t4, 20 # VPN[1] * PTESIZE.
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srli t4, a0, 20 # VPN[1] * PTESIZE.
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add t4, t0, t4 # a + VPN[1] * PTESIZE.
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srli t3, t2, 2
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ori t3, t3, 0b1
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@@ -72,6 +154,8 @@ imem_mirror:
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# Parameters:
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# a0 - Physical start of the text section.
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# a1 - Page aligned address following the kernel (kernel end).
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# a2 - Offset from the physical to virtual memory.
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# a3 - Size of additional page tables.
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.type imem_initialize, @function
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imem_initialize:
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# Prologue.
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@@ -80,16 +164,26 @@ imem_initialize:
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sw s0, 24(sp)
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addi s0, sp, 32
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sw a1, 20(sp)
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sw s1, 16(sp)
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sw s2, 12(sp)
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sw s3, 8(sp)
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# Set s1 to the beginning of free memory (skipping root page table).
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li s1, 4096
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add s1, a1, s1
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slli a3, a3, 1 # Multiply by two to account for the copy in virtual memory.
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# s1 is a1 + PAGE, where PAGE is a free page reserved for future use,
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# since the size of the root page table is already included in a3.
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# s1 is used to avoid calculating a1 + PAGE_SIZE twice.
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add a3, s1, a3
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sw a3, 20(sp)
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# Set boundaries between segments.
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lui s2, %hi(etext)
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sub s2, s2, a2
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lui s3, %hi(_data)
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sub s3, s3, a2
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# Set the root page since it is used by mapping functions,
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# but don't activate paging before the segments are property mapped.
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@@ -115,8 +209,6 @@ imem_initialize:
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# Data section.
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addi a0, s3, %lo(_data)
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lw a1, 20(sp)
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li t0, 4096 * 3
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add a1, a1, t0
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mv a2, s1
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li a3, 1
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li a4, 0b0110
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