• Jeremy Fitzhardinge's avatar
    x86: add brk allocation for very, very early allocations · 93dbda7c
    Jeremy Fitzhardinge authored
    Impact: new interface
    Add a brk()-like allocator which effectively extends the bss in order
    to allow very early code to do dynamic allocations.  This is better than
    using statically allocated arrays for data in subsystems which may never
    get used.
    The space for brk allocations is in the bss ELF segment, so that the
    space is mapped properly by the code which maps the kernel, and so
    that bootloaders keep the space free rather than putting a ramdisk or
    something into it.
    The bss itself, delimited by __bss_stop, ends before the brk area
    (__brk_base to __brk_limit).  The kernel text, data and bss is reserved
    up to __bss_stop.
    Any brk-allocated data is reserved separately just before the kernel
    pagetable is built, as that code allocates from unreserved spaces
    in the e820 map, potentially allocating from any unused brk memory.
    Ultimately any unused memory in the brk area is used in the general
    kernel memory pool.
    Initially the brk space is set to 1MB, which is probably much larger
    than any user needs (the largest current user is i386 head_32.S's code
    to build the pagetables to map the kernel, which can get fairly large
    with a big kernel image and no PSE support).  So long as the system
    has sufficient memory for the bootloader to reserve the kernel+1MB brk,
    there are no bad effects resulting from an over-large brk.
    Signed-off-by: default avatarJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
    Signed-off-by: default avatarH. Peter Anvin <hpa@zytor.com>
head32.c 1.16 KB