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#include <fbos/image.h>
#include <fbos/mm.h>
.global _start
.global _start_kernel
.section .head.text
// Since this kernel boots in virtualization environments and machines that are
// expecting a Linux kernel, we have to adhere to how bootloaders expect Linux
// kernel images to look like. In particular, the entry of the kernel is
// actually a Boot image header, as defined in:
//
// https://docs.kernel.org/arch/riscv/boot-image-header.html
//
// Conveniently, for machines who don't care about this (e.g. QEMU), the first
// 64 bits correspond to two instructions that can be set up, and hence entering
// here will simply jumpt to `_start_kernel`, our real entry.
_start:
// The first two words give us room for two executable instructions. Linux
// uses that on EFI support to first allocate a magic value for UEFI and
// then have a `j _start_kernel` instruction. Otherwise it just allocates
// the first one for `j _start_kernel` and leaves the second word empty. The
// latter is what we do here as well.
j _start_kernel
.word 0
// Ensure alignment for the next double word.
.balign 8
// Load offset. Note that this matches the LOAD_BASE_OFFSET as defined in
// `include/mm.h`.
.dword LOAD_BASE_OFFSET
// Size of the image. This is *mandatory* as per bootloader request.
.dword _end - _start
// Flags. As defined by Linux, only one bit matters here, which is related
// to endianness. Setting 0 means little-endian.
.dword 0
// Header version.
.word RISCV_HEADER_VERSION
// Reserved fields.
.word 0
.dword 0
// Deprecated image magic.
.ascii RISCV_IMAGE_MAGIC
.balign 4
// Good image magic, in little-endian format.
.ascii RISCV_IMAGE_MAGIC2
// Reserved field.
.word 0
_start_kernel:
// Mask all interrupts
csrw sie, zero
csrw sip, zero
// Flush the instruction cache
fence.i
// Run the hart lottery. If this is not the first time that it happens, then
// stall this hart forever: on this simple kernel we only want one hart
// available to avoid SMP shenanigans. See explanation on fbos/init.h.
la a3, hart_lottery
li a2, 1
amoadd.w a3, a2, (a3)
bnez a3, .Lhart_wait
// Explicitely nullify the 'gp' and 'sscratch' registers, as they are a bit
// special but we are not using them. For the rest of the registers, we
// explicitely do not care to reset them.
li gp, 0
csrw sscratch, 0
// Point 'tp' to the init task. The 'tp' register will always point to the
// current process being executed, and it will be shown on debug when
// printing out messages.
la tp, tasks
// Point 'sp' the our global stack. See fbos/sched.h for more details.
la sp, stack + STACK_SIZE
// Start the kernel. Notice that both 'a0' and 'a1' have been left
// untouched. This is no coincidence as the values as passed from the
// bootloader for both these two registers will be passed down to the kernel
// as is. As with the Linux kernel these two registers contain:
// - a0: boot hart id.
// - a1: pointer to the flattened device tree blob.
tail start_kernel
// We really shouldn't reach this point. If so, at least mask again all
// interrupts, flush the instruction cache and halt the current hart.
csrw sie, zero
csrw sip, zero
fence.i
// This is where harts come to die :)
.Lhart_wait:
wfi
j .Lhart_wait
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