• Mark Rutland's avatar
    arm64: Update the Image header · a2c1d73b
    Mark Rutland authored
    Currently the kernel Image is stripped of everything past the initial
    stack, and at runtime the memory is initialised and used by the kernel.
    This makes the effective minimum memory footprint of the kernel larger
    than the size of the loaded binary, though bootloaders have no mechanism
    to identify how large this minimum memory footprint is. This makes it
    difficult to choose safe locations to place both the kernel and other
    binaries required at boot (DTB, initrd, etc), such that the kernel won't
    clobber said binaries or other reserved memory during initialisation.
    
    Additionally when big endian support was added the image load offset was
    overlooked, and is currently of an arbitrary endianness, which makes it
    difficult for bootloaders to make use of it. It seems that bootloaders
    aren't respecting the image load offset at present anyway, and are
    assuming that offset 0x80000 will always be correct.
    
    This patch adds an effective image size to the kernel header which
    describes the amount of memory from the start of the kernel Image binary
    which the kernel expects to use before detecting memory and handling any
    memory reservations. This can be used by bootloaders to choose suitable
    locations to load the kernel and/or other binaries such that the kernel
    will not clobber any memory unexpectedly. As before, memory reservations
    are required to prevent the kernel from clobbering these locations
    later.
    
    Both the image load offset and the effective image size are forced to be
    little-endian regardless of the native endianness of the kernel to
    enable bootloaders to load a kernel of arbitrary endianness. Bootloaders
    which wish to make use of the load offset can inspect the effective
    image size field for a non-zero value to determine if the offset is of a
    known endianness. To enable software to determine the endinanness of the
    kernel as may be required for certain use-cases, a new flags field (also
    little-endian) is added to the kernel header to export this information.
    
    The documentation is updated to clarify these details. To discourage
    future assumptions regarding the value of text_offset, the value at this
    point in time is removed from the main flow of the documentation (though
    kept as a compatibility note). Some minor formatting issues in the
    documentation are also corrected.
    Signed-off-by: default avatarMark Rutland <mark.rutland@arm.com>
    Acked-by: default avatarTom Rini <trini@ti.com>
    Cc: Geoff Levand <geoff@infradead.org>
    Cc: Kevin Hilman <kevin.hilman@linaro.org>
    Acked-by: default avatarWill Deacon <will.deacon@arm.com>
    Signed-off-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
    a2c1d73b
booting.txt 8.63 KB