HAL for RISC-V 64 based targets#

HAL for RISCV64 architecture is located in src/hal/riscv64.

Initialization#

Kernel execution starts from _start symbol located in _init.S file.

    _start:
        /* Mask all interrupts */
        csrw sie, zero

        /* Disable FPU */
        li t0, SR_FS
        csrc sstatus, t0

First instructions mask interrupts and disable FPU.

        /* Initialize syspage */
        la a0, syspage
        la t0, pmap_common
        li t1,  3 * SIZE_PAGE /* pdirs */
	addi t1, t1, SIZE_PAGE /* stack */
	addi t1, t1, SIZE_PAGE /* heap */
        add t0, t0, t1
        sd t0, (a0)

The next step is setting the syspage pointer.

        call dtb_parse

The hardware configuration is passed to kernel using Device Tree Blob (DTB). Pointer to DTB is stored in a1 register. DTB parser extracts information necessary to kernel from tree and stores it in kernel structure. It could be used later by other components using dtb_ functions.

        call _pmap_preinit

After evaluating hardware configuration initial kernel page tables are initialized.

        li a1, VADDR_KERNEL
        la a0, _start
        li t0, 0xffffffffc0000000
        and a0, a0, t0
        sub a1, a1, a0

The relocation offset is calculated.

        /* Relocate stack */
        la sp, pmap_common
        li t0, 3 * SIZE_PAGE /* pdirs */
	addli t0, t0, SIZE_PAGE /* stack */
        add sp, sp, t0
        add sp, sp, a1

        /* Relocate syspage */
        la a0, syspage
        ld t0, (a0)
        add t0, t0, a1
        sd t0, (a0)

And relocation of stack and syspage is performed.

        /* Point stvec to virtual address of intruction after satp write */
        la a0, 1f
        add a0, a0, a1
        csrw stvec, a0

        la a0, pmap_common
        srl a0, a0, 12
        li a1, 0x8000000000000000
        or a0, a0, a1

        sfence.vma
        csrw sptbr, a0

The above sequence enables paging and pass execution to proper virtual address by setting the trap vector.

        /* Add dummy page fault trap handler */
        la a0, .Lsecondary_park
        csrw stvec, a0

        call main
        li a7, SBI_SHUTDOWN
        ecall

.Lsecondary_park:
        wfi
        j .Lsecondary_park