RISC-V Linux kernel memory initialization on boot.

This text is based on memory-initialization.md from my GitHub repo  riscv-notes

The kernel is started with virtual memory initialized by machine level bootloader BBL. The more detailed description can be found in this document - supervisor_vm_init.md .

The kernel start offset is defined in linux/linux-4.6.2/arch/riscv/include/asm/page.h

/*
 * PAGE_OFFSET -- the first address of the first page of memory.
 * When not using MMU this corresponds to the first free page in
 * physical memory (aligned on a page boundary).
 */
#ifdef CONFIG_64BIT
#define PAGE_OFFSET     _AC(0xffffffff80000000,UL)
#else
#define PAGE_OFFSET     _AC(0xc0000000,UL)
#endif

BBL initializes virtual memory for supervisor mode, maps the Linux kernel at PAGE_OFFSET, sets sptbr register value to a root page table physical address, switches to the supervisor mode with $pc set to the entry point _start. BBL does this in enter_supervisor_mode function defined in riscv-tools/riscv-pk/machine/minit.c

void enter_supervisor_mode(void (*fn)(uintptr_t), uintptr_t stack)
{
  uintptr_t mstatus = read_csr(mstatus);
  mstatus = INSERT_FIELD(mstatus, MSTATUS_MPP, PRV_S);
  mstatus = INSERT_FIELD(mstatus, MSTATUS_MPIE, 0);
  write_csr(mstatus, mstatus);
  write_csr(mscratch, MACHINE_STACK_TOP() - MENTRY_FRAME_SIZE);
  write_csr(mepc, fn);
  write_csr(sptbr, (uintptr_t)root_page_table >> RISCV_PGSHIFT);
  asm volatile ("mv a0, %0; mv sp, %0; mret" : : "r" (stack));
  __builtin_unreachable();
}

The important difference between RISC-V case and many other CPUs( e.g. x86 )is that Linux kernel's entry point is called with virtual memory initialized by boot loader executing at higher privilege mode.

The memory management is initialized inside setup_arch routine defined in linux/linux-4.6.2/arch/riscv/kernel/setup.c, below only memory management relevant part of the function is shown

void __init setup_arch(char **cmdline_p)
{
...
    init_mm.start_code = (unsigned long) _stext;
    init_mm.end_code   = (unsigned long) _etext;
    init_mm.end_data   = (unsigned long) _edata;
    init_mm.brk        = (unsigned long) _end;

    setup_bootmem();
    ....
    paging_init();
    ....
}

The _stext, _etext, _edata, _end global variables are defined in the linker script linux/linux-4.6.2/arch/riscv/kernel/vmlinux.lds.S which defines the kernel memory layout. These variables defines the kernel section borders. The thorough description regarding linkers scripts can be found here https://sourceware.org/binutils/docs/ld/Scripts.html .

The first function being called is setup_bootmem

static void __init setup_bootmem(void)
{
    unsigned long ret;
    memory_block_info info;

    ret = sbi_query_memory(0, &info);
    BUG_ON(ret != 0);
    BUG_ON((info.base & ~PMD_MASK) != 0);
    BUG_ON((info.size & ~PMD_MASK) != 0);
    pr_info("Available physical memory: %ldMB\n", info.size >> 20);

    /* The kernel image is mapped at VA=PAGE_OFFSET and PA=info.base */
    va_pa_offset = PAGE_OFFSET - info.base;
    pfn_base = PFN_DOWN(info.base);

    if ((mem_size != 0) && (mem_size < info.size)) {
        memblock_enforce_memory_limit(mem_size);
        info.size = mem_size;
        pr_notice("Physical memory usage limited to %lluMB\n",
            (unsigned long long)(mem_size >> 20));
    }
    set_max_mapnr(PFN_DOWN(info.size));
    max_low_pfn = PFN_DOWN(info.base + info.size);

#ifdef CONFIG_BLK_DEV_INITRD
    setup_initrd();
#endif /* CONFIG_BLK_DEV_INITRD */

    memblock_reserve(info.base, __pa(_end) - info.base);
    reserve_boot_page_table(pfn_to_virt(csr_read(sptbr)));
    memblock_allow_resize();
}

The Linux kernel queries the available memory size in setup_bootmem by invoking SBI interface's sbi_query_memorywhich results in a call to __sbi_query_memory BBL routine executed (suprisingly) in supervisor mode as SBI has been mapped to the supervisor virtual address space and ecall instruction is not invoked for sbi_query_memory

uintptr_t __sbi_query_memory(uintptr_t id, memory_block_info *p)
{
  if (id == 0) {
    p->base = first_free_paddr;
    p->size = mem_size + DRAM_BASE - p->base;
    return 0;
  }

  return -1;
}

More about SBI can be found here https://github.com/slavaim/riscv-notes/blob/master/bbl/sbi-to-linux.md .
The kernel reserves the pages occupied by the kernel with a call to memblock_reserve(info.base, __pa(_end) - info.base); . Then a call to reserve_boot_page_table(pfn_to_virt(csr_read(sptbr))); reserves the pages occupied by the page table allocated by the bootloader, i.e. BBL.The Linux kernel retrieves the page table allocated and initialized by BBL by reading a physical address from the sptbr register and converting it to a virtual address. The page table virtual address is also saved at the master kernel Page Tables init_mm.pgd. The snippet is from linux/linux-4.6.2/arch/riscv/mm/init.c

void __init paging_init(void)
{
    init_mm.pgd = (pgd_t *)pfn_to_virt(csr_read(sptbr));
  ....
}